Your best bet is to look at all the games that have been written in Ruby/Gosu and see what they achieve. As jlnr points out, most are unfinished, and it might be hard for you to see which ones really push the envelop of Ruby/Gosu development. You might like to try running my
Wrath game, for example. Pressing F in-game will show the FPS and potential FPS in the overlay (that is, the FPS the game would achieve if the frame rate wasn't capped at 60).
One thing I'd recommend is for you to do a simple rendering test of your game idea and see what the frame-rate is like. That is as far as I've got with
Smash and Grab, which achieves a nice 400 FPS, though of course you need to allow for the fact that the game is extremely light on game-logic at this point.