This is a source gem, that needs the Ruby DevKit installed on the end user system, because it needs to be compiled at install time. But, SketchUp embedded Ruby is not yet set up to be hooked into the Ruby DevKit. So, gems that need compiling are a problem for SketchUp.
Choices, are:
(A) to install both the 32-bit and 64-bit Ruby DevKits and pre-compile the gem for each platform and bitness, and distribute them with your plugin code, using a conditional expression to choose the correct (platform and bitnees) compiled library to load into SketchUp’s embedded Ruby.
(B) find already pre-compiled binaries of the library, and package them with your plugin.
(C) find alternate pure-ruby library gem(s) that do not need compiling, and therefor are already cross-platform.
I have been recommending (C), the Pure-Ruby gems that need no compiling.
See:
Pure Ruby replacement libraries for the “mysql” gem