How Do I Build This? (Graviton Pulse Field/Graviton Surge)

[Varyon]

First off, I should note that to my knowledge, Area Effect and Rapid Fire aren't technically compatible, you'd need to make the attack Explosive instead (which has some pretty harsh fall-offs with distance, but on the bright side can cause shrapnel damage by default via incidental fragmentation - see B415). I'd suggest Explosive 3, as that has the least-harsh fall-off. Honestly, the Rapid Fire part of it looks a lot like it's meant to be a way to simply reduce cost, but I could be mistaking your intent.

Originally Posted by DoctorJerk

1) Should the crushing damage be summed before knockback is calculated, or for each tick separately?

I have no problem with summing up all the knockback generated by the attack to assess the effect. Arguably, they shouldn't combine linearly like this, but for simplicity it's probably acceptable*. From a game-balance perspective, the primary difference between assessing them separately is that you end up dropping fractions in that case (although there's also velocity to keep in mind).

*Consider two people with ST 12 pushing you, when compared to one person with ST 17 - which is twice as strong by BL - pushing you. Realistically, the two scenarios would result in the same amount of knockback. An ST 12 push averages 5 knockback, for 10 knockback with two people. An ST 17 push averages 11 knockback, which is close enough to be fine. However, for 2xST 17 vs ST 24 (again, twice as strong by BL; note I'm using odd numbers here to get as close to "double BL" as possible), you're looking at 22 knockback vs only 16.
If you want to be a glutton for complication, an option would be to find out what ST score corresponds to the knockback of each hit, then do a sum of squares - that is, square the ST scores, add them together, then take the square root - to get the "combined ST," then use that for calculating knockback. So, for 10 hits from a 2d attack, 2d corresponds to thr for ST 21 or 22; we'll use 21.5. 21.5^2=462.25, or 4622.5 when multiplied by 10. 4622.5^0.5 is 68, which is around 8d-1 (rather than the 20d you'd calculate normally). I'd recommend against this approach, in no small part because it's an attempt to be realistic, but is reliant on the rather-unrealistic ST-based damage table.

Originally Posted by DoctorJerk

2) Does this ability even need the Cosmic: Irresistible Attack enhancement (Powers, P.128)?

The fact it's a knockback-only attack gets rid of the bulk of the benefit of Irresistible Attack - personal DR doesn't matter to those, although Cover DR from intervening walls and the like does. If I were GM, I'd drop the cost of Irresistible Attack to only +50% (and possibly rename it to "Ignores Walls") for this power. If you don't want it to affect your allies (and yourself), I'd strongly suggest Selective Area +20% (or maybe just Selectivity +10%, so that you can turn off "Ignores Walls" when that would be inconvenient).

Originally Posted by DoctorJerk

3) Does the Very Rapid Fire enhancement remove one's ability to determine how many shots to fire?

I see no reason why it would; so far as I can tell, it's really just an Enhancement that reduces Rcl to 0.5, but explains this in a roundabout fashion because non-integer Rcl isn't a thing in GURPS. That being said, the existence of the Selective Fire Enhancement as part of Rapid Fire implies that Rapid Fire normally doesn't let you pick how may shots you take - or, at least, it assumes all RoF 5+ powers have RoF n! if they lack Selective Fire (which means minimum number of shots is 1/4 RoF).

Originally Posted by DoctorJerk

4) If an enemy is in the center of the area of effect and pressed against another object being affected by this attack, should they take collision damage based on how fast each object would have been moving toward each other? Basically, the victims are being mashed together by a powerful force, surely that should produce a damaging effect.

Something at the epicenter shouldn't take actual damage from anything other than other stuff impacting against it. It's a lot easier to hurt someone by shoving them into a wall than by shoving them toward it when they're already right up against it. Arguably, there would be some effect of basically trying to crush them, but this would be reduced compared to the damage from flinging them into walls. If you want to still have there be some damage at the epicenter, an option (albeit a somewhat-complex one) might be to only have No Wounding apply to the Area Effect/Explosive part of the attack and perhaps some of the basic attack itself. Let's say whatever is at the epicenter gets crushed for half the basic damage, but those away from it simply get flung toward the center. Using Explosive 3, you'd apply No Wounding and No Blunt Trauma to that (via the rules for Limiting Enhancements), reducing it from +150% to +30%, and to half the damage of the attack itself, which works out to "Half Wounding" being -35%. Irresistible Attack is no longer justified as being quite so cheap, however; I'd argue that Irresistible Attack applies at full cost to half the damage (which works out to +150%), and I'd just ignore its cost for the other half.

Originally Posted by DoctorJerk

How would you model it if you wanted to create an attack that produced a continuous gravitational pull towards a central location, something you'd have to try to resist even if passing through for only a moment? e.g Zarya's Graviton Surge ability from Overwatch

Give it Persistent +40% - which I see you've actually already done. This is technically only available for Area Effect, but I don't think it would be a problem to let it apply to Explosive as well. It does exactly what you want - anyone who passes through the area is automatically hit by the attack, and those within it are affected until they're able to escape or the effect ends. There is the oddity that it technically lets any character move their full Move on their turn, and then if they didn't escape the pull it yanks them back in, but that's up to the GM to address (personally, I'd let you do your normal Move, then assess the knockback immediately, and decide wherever you end up is actually where you managed to move to, with no yo-yo'ing involved; if the knockback resulted in the character falling, I'd be tempted to have their margin of failure dictate how far they got before losing their balance).

...

All told, your ability might look something like the following. Note enhancing Range (these are all Range 10/100) and Accuracy (these are all Acc 3) will, of course, increase the price.

Innate Attack 5d cr (Cosmic: Ignores Walls +50%; Double Knockback +20%; Explosion 3 +150%; Persistent +40%; Rapid Fire (x10) +100%; Reverse Knockback +20%; Very Rapid Fire +10%; Elemental: Gravity -10%; No Blunt Trauma -20%; No Wounding -50%) [93]

When the attack hits a target, it creates an implosion field at the point of impact. Anything not at the point of impact is pulled toward it, doing 5dx2 of knockback divided by the distance from the epicenter in yards. The implosion field deals no damage on its own, but objects within it may collide for normal collision damage, and if there is any debris around the victims may be struck by it (see incidental fragmentation, B415). The field lasts for 10 seconds.

Alternatively, if you want Area of Effect (so that it affects everyone within a given area equally), you'll need to give up Rapid Fire and Very Rapid Fire, and use the savings for additional damage, although you don't get much.

Innate Attack 5d+1 cr (Area Effect 16 yards +200%; Cosmic: Ignores Walls +50%; Double Knockback +20%; Persistent +40%; Reverse Knockback +20%; Elemental: Gravity -10%; No Blunt Trauma -20%; No Wounding -50%) [93]

If you liked my complicated idea for the epicenter doing some damage, here's the way that build would work, going off of the Area Effect version.

Innate Attack 5d (Area Effect 16 yards (No Blunt Trauma -20%; No Wounding -50%) +60%; Cosmic: Irresistible Attack (Half Damage, Epicenter Only, otherwise Ignores Walls, -50%) +150%; Double Knockback +20%; Persistent +40%; Reverse Knockback +20%; Elemental: Gravity -10%) [95]