Go to the 'Forums' section, and then go to 'Performance', and then to 'Engine Swap Forum', and read the threads on swapping an engine into a GT. You can read on all the plusses and minuses of swapping a V8 into a GT. Yes, it's been done in the past, but the success rate of most people who set out to put a small block Chevy in is around 5-10%.
It's a lot of work, and will require another tranny, another driveshaft, another rear axle, bigger brakes, a much bigger radiator, a different front spring, cutting the firewall, custom engine mounts, and installing a rollcage. Why a rollcage? Every V8 swap I've seen that did NOT have a cage broke windshields every time they stomped on the throttle, or popped rear windows out of the body...the chassis will flex....a lot.
But if you are inquiring as to whether there's a kit or instructions on how to swap a Chevy V8 into a GT, there's not. The most common swaps seem to be the GM 60 degree V6, or Mazda rotary engine. There IS a substantial amount of information out there on either of these swaps. I suggest you do your homework and read up on them all you can, because it will prepare you for the kind of work you may have ahead of you.
While I don't necessarily care for engine swaps in Opels (hey, it's a free country!), I can appreciate a well-done swap. A good swap will require more work in planning before the swap starts than the swap itself takes to do. If you take the time to do it right, then it'll be worth it, otherwise you'll probably lose interest and the car will go to the crusher one day half finished!
Good luck.
Bob