[Rupert]

Whether a grenade stops where it lands or bounces depends on many things, such as whether it was thrown hard or lobbed, and what ground it lands on. If it's soft forest humus, it'll just stop, but on hard concrete it'll bounce and keep going. Perhaps unfortunately this is a GM's call.

As for the timer, for those 20th century grenades given as having 4-5s fuses, that's being generous - they actually had more like a 4-7s delay in service.

If flight time really matters, I'd rule that a throw of under half the thrower's maximum distance lands at the end of their current turn, and one over that lands at the beginning of their next. The fuse would 'tick' at the start of their turn, so a 1s fuse can't be safely 'cooked off' - it'll go off in your hand.

By the way, in my experience grenades are very slow weapons in GURPS and are of little use unless one or both sides of a firefight are in solid cover and can't be forced out or shot in place. It takes a long time to get the grenade out, prep it, and throw it, and accurate placement is hard without exposure to fire.