Gosu 0.9.2 and 0.8.7.2 have the same rendering backend. Even Gosu 0.7.x, which didn't use libSDL to open the window, used effectively the same OpenGL code internally. But still, go ahead and benchmark all the combinations you are interested in, maybe there are a few surprises :)

FWIW I saw this question on StackOverflow a (long) while ago: http://stackoverflow.com/questions/1784138/why-rubygame-and-gosu-are-slower-than-pure-opengl

For my fighting game prototypes I have not had any performance issues even on my 1st gen netbook but the code is also pretty lean.