Without ruby20-gems and ruby21-gems in the official pkg repo you can't install any gems, unless you compile devel/ruby-gems with the correct $DEFAULT_VERSIONS, if you are running ruby20/ruby21. This works quite well for php extensions, but they have separate ports for each php version.
Auto-assigned to maintainer ruby@FreeBSD.org
This is true, someone needs to work on changes to bsd.ruby.mk to make this happen. Perhaps using bsd.python.mk as a guide.
For the record, there is work going on the support flavours in the ports tree in general and I think this is the correct solution, rather than ruby specific changes to support this.
Flavor support in ports is making progress, and I still think that's the right solution rather than something ruby specific. So closing for now, but the issue isn't forgotten and I'll test rubygem stuff when flavor support when it arrives.