GameStop's digital distribution service is essentially Impulse rebranded, so it's entirely dependent on the game as far as trainer compatibility goes. I would honestly say it might be better to only buy games that require Steam, if you're worried about GS.
Darksiders II is going to require Steam, at least - so I imagine there will probably only be one version of the trainer, which should work for you (and I). Pure speculation on the trainer since the game isn't out of course, as the game could unexpectedly require eighteen different always-on DRM processes running with built in anti-cheat measures.
Either or - I got the game throgh gamestop.com, and yea you guys are right - requires STEAM. You do use the Gamestop download manager, but thats to download just a basic, 6MB install with the serial/product key. I'm pre-loading it on STEAM right now. So yay, get to use the STEAM trainer! Thanks for your help guys!