That is not correct - SketchUp do not support the gem system. We strongly recommend against using it in production extensions. Embedding it into your own extension namespace is the way to go.
More details: Install Prawn gem in SketchUp 2016 - #2 by tt_su
There seem to be more and more questions about usage of gems. I’m almost tempted to introduce a warning to SU’s version of Gem.install to make it clear that it’s not a supported or recommended feature.
Side note, if you use rubocop-sketchup to analyse your extension code you get a lot of recommendations and suggestions. It also cover a number of cases where we might reject an extension to the Extension Warehouse. Gem.install is one of these: Requirements Cops - RuboCop SketchUp: SketchUp Extension Best Practices