<p>If I download a .gem file to a folder in my computer, can I install it later using <code>gem install</code>?</p>

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.