If I download a .gem file to a folder in my computer, can I install it later using gem install?
Yup, when you do gem install, it will search the current directory first, so if your .gem file is there, it will pick it up. I found it on the gem reference, which you may find handy as well:
gem install will install the named gem. It will attempt a local installation (i.e. a .gem file in the current directory), and if that fails, it will attempt to download and install the most recent version of the gem you want.