gGo can be used as client for GNU Go or any AI that supports GTP to play against the computer. Previously saved games can be resumed.
You require GNU Go 3.0 is newer. Older versions do not support GTP.
Download GNU Go 3.6 for Windows (sources) or check this page for alternative packages.
Old version: GNU Go 3.4 (sources)
Download GNU Go 3.6 for Mac.
Install GNU Go from your distribution (most Linux distributions include it), or visit the GNU Go Homepage.
To let gGo find GNU Go, make sure the GNU Go binary is in your PATH (should be anyways on Linux), or tell gGo where the binary can be found in the gGo preferences.
On Linux this would for example be: /usr/local/bin/gnugo
On Windows something like: C:\Program Files\GnuGo\gnugo.exe
Note you need to include the binary and not only give the directory.
For a more detailed explanation please see the FAQ.