The C++ version of Gosu is mostly an elaborate wrapper around the SDL 2 library, and is suitable for commercial games on macOS, iOS, Windows, and Linux. I used to have a commercial Gosu game on the iOS App Store and never had any issues.

The RubyMotion version of Gosu for iOS and macOS seems to work well enough, but nobody has tested it on the App Store as far as I know.

The 'normal' Ruby/Gosu library on Windows, macOS, and Linux suffers from the fact that Ruby can't really be compiled. This should be fixable with mruby, but nobody has put in the work yet.
The typical ways of distributing Ruby/Gosu games, i.e. using Ocra on Windows and the Ruby.app wrapper on macOS, do not hide or obfuscate your source code at all. That doesn't technically stop you from selling these games, but neither way looks super professional.

In terms of bugs, there are rare reports where games don't work on some Windows machines, but it's extremely hard to determine if those are issues with the graphics drivers (specifically OpenGL) or with Gosu/SDL 2.

All of that being said, I'm always willing to prioritize issues if it helps anyone ship games, especially commercial ones.