I have an '03 GP GT.
how much HP can the stock trans handle?
when shopping for a new one, is there any specific type I need to look for?

It all depends how you drive it. But in the short long run, it won't take a beating. I believe you have the 4T65e, the stronger is the 4T65e-HD. A buddy in another club had a turbo on his GT, wasn't tuned right, but the tranny didn't last long. If you are serious you can pump some serious horsepower, but if you are planning on doing that, it would be wise to get an HD kit for your tranny or get some tranny parts that are more beefier, ZZPerformance.com

theres not a HUGE difference between the HD and non-HD. i think some people have a misconception that they're beefier all around, but they really aren't... just a few things like the differential.

but basically anything more than light-medium mods on these heavy cars are going to wear down the tranny and eventually kill it.
If you plan on serious power, your engine will handle it fine (properly tuned) but you'd better plan for a ZZP heavy duty tranny.

ok. Thx guys