having an elgato or any capture device....
do not use it for:
Gaming PC > elgato > Gaming PC = too much hardware usage and useless, pc has gaming+elgato+obs usage.
You are better off simply just recording from OBS and nothing else.
use it for:
Gaming PC > elgato > Streaming PC = gaming pc is free and only the streaming pc has the elgato+obs usage
Console > elgato > PC = pc has the elgato+obs usage
I tried game capture without the Elgato, but all I get is a black screen then (I'm trying to stream an ARMA 2 mod, maybe that's the black screen problem).
That black screen with game capture is that OBS needs a fixing to detect that game, however you could try testing running OBS as an Administrator and then testing game capture to see if its shown(also test enabling in game capture options, anti-cheat compatibility hooking). Test both 32 bits OBS and 64 bits OBS just in case if only on one of them it works capturing it. If nothing then report it as a bug, of not detecting Arma 2 through Game Capture.
The other suggestion that should work is if you have tried running the game in windowed mode, and then use Window Capture? That should not fail if you are on Windows 8. Something minor make sure you have installed on your computer Directx 9c.