I had the problem sitting there for a long time, and I finally found the solution this morning.
When I was launching Vim from the terminal, my $PATH variable was setup properly.
Idem when I was launching Gvim from the terminal, all good.
But when I launching Gvim from the launcher, the $PATH always ended up to the default $PATH (usual /usr/bin … but no trace of my rvm installation). Causing rails.vim and some other commands, Ruby related, to fail.
One of the solution is to edit the Gvim launcher with the command:
bash -lc Gvim
I’m happy I found it, that will give the opportunity to have a proper look at Gvim as a terminal alternative. I found that having menu can be useful time to time for plugins that you don’t use often.