The gem you would like to use (google-protobuf) is often referred to as an ‘extension gem’. @ene_su mentioned them above as a gem that ‘requires local compiling’.
An easy way to determine if they are extension gems is to look at the gem’s ‘all versions’ page at rubygems.org and see if they distribute gems with descriptions like ‘x64-mingw32’ or ‘universal-darwin’. If not, one has to look at the git repository and check for ‘non-ruby’ code, typically this would be c, c++, and/or java code, depending on what’s supported.
Anyway, these gems are very difficult to embed in a SU extension/plugin, especially one that can be used in Windows and macOS. Also, Windows issues with SU using MSYS2 mingw Ruby in previous versions, and using MSVC/mswin Ruby in more recent ones.
It can be done, but it requires knowledge of how to build extension gems, and they can only be built with external stand-alone Ruby versions…