I would probably use the BT mech creation system and figure out a conversion from those stats to GURPS stats. Something like this:
- Use the decade damage scale (1dHP = 10 HP)
- Rule 1 point of BT internal structure is 5 dHP
- Use BT armor values and say they are all semi-ablative DR
- Use hit locations as normal in GURPS
- Convert BT speeds in hexes/turn to yards/second
- Generate GURPS stats for all the BT weapons; i.e. say an AC2 is: dmg 2d(3), acc 4, range 3000/10000, ROF 8, etc. Here you can either try to closely follow the BT stats, or make stats more in line with Ultratech.
- You might want to say vehicular combat takes place on a larger scale with longer turns, like spaceship combat in GURPS Traveller, but figuring all that out could be a lot of work.
Now that will be some work, and afterwards you will have to look at the numbers to make sure there is some balance.