How do you return from 'gf' in Vim

I am using Vim for windows installed in Unix mode. Thanks to this site I now use the gf command to go to a file under the cursor.

I'm looking for a command to either:

1. return to the previous file (similar to Ctrl+T for ctags), or
2. remap gf to automatically launch the new file in a new window.

I use Ctrl-O