If for some reason, the project cannot use bundler and gem install, we can use
script/plugin install git://github/<author name>/ ... /foo.git
but what if it is not on github and for sure it is a ruby gem, can script/plugin install install from the rubygems.org which is the official gems place? (that's the one gem install uses, isn't it?)
script/plugin install can only install plugins from git repositories, not gems. However many gems are similar in structure and function to plugins, so you can try installing them as plugins like that and they may work. If the gem has its source on github then it would be easy. Otherwise if you can get ahold of the source of a gem, you can copy it into your plugins folder (just reference existing plugins to see where they go).