Here is how you can use gems in your gemfile by specifying the path to the file.

Setting the path to the plain gem that doesnt seem to work and you will get errors like:
The path `/path/to/paperclip` does not exist.
That is because the :path thing actually wants you to point it to a folder where the gem is already installed. Do the following:

gem install /path/to/paperclip-3.0.0.gem
updatedb
locate paperclip
cp -r -v /home/username/.rvm/gems/ruby-1.9.3-p194/gems/paperclip-3.0.0 /your/app/root/some_dir/

Now you can specify it in the Gemfile as follows:
gem “paperclip”, :path => “/your/app/root/some_dir/paperclip-3.0.0”