While this tutorial should give you a good overview of Gerris, it is by no means a complete description. To learn more you should first consult the Gerris Frequently Asked Questions and the Gerris reference manual which describes each object and the corresponding file parameters in more detail.
You should also have a look at the Gerris Examples page for examples of how to use Gerris for a range of problems. The parameter files are cross-linked with the reference manual.
Another source of more advanced examples is the Gerris test suite.
If things are still unclear you can ask for help on the gfs-users mailing list. Please note that you first need to subscribe to the list to be able to post messages.