Gosu uses the well-maintained SDL library for creating the window and setting up OpenGL. If vsync doesn't work on your Ubuntu system, then it's not a problem with Ruby/Gosu, but with all SDL-based games.
There was someone on IRC just this week who also experienced tearing (lack of vsync) on Linux with Gosu. I'm afraid vsync issues on Linux will be with us until everyone uses Wayland (which Nvidia's drivers don't support, yay).
https://unix.stackexchange.com/a/182841The only Gosu-specific advice I can give you is to keep the
update_interval
at its default value, in which case the vsync on Windows should not slow your game down.