Is there any reason these are being built from the .gemspec's via git? I thought they were basically converted to a single file .gem, kinda like a tarball of the sources with metadata.

Is there any guideline how ruby gems should be handled? Here it downloads the gem and then installs it, downloading the tarball works as well.

No, hence asking questions since I don't know ruby (outside of creating the macros and splitting vagrant). To me it seems like the more official builds seem to be via rubygems, where git is the development. 95% of the time these might be the same. These will be what you get when you install the packages via ruby so unless there is a reason to do otherwise, it seems the most logical option.