Which version of GEM are you using? All of the ones built in the last 6 months or so would have pix_video aliased to pix_videoDarwin. Does pix_video (or videoDarwin) print any errors to the console? I know that it will print a set of messages when the object is first loaded, these happen if certain processes are successful or fail.
GEM: ver: 0.87+cvs
I am compiling the latest version of GEM right now. I hope it will fix the problem.