There is a variety of documentation available on using Gramps.
The comprehensive User manual is available in English, French, Dutch and Slovak.
The FAQ answers some of the most common questions asked about Gramps.
Some Reviews provide a critical perspective on Gramps.
There is a detailed guide on how to use Gramps for Recording UK Census data. Note that this covers how to record sources, so is useful for other countries as well.
If you have any questions on Gramps, then the Mailing Lists are a good place to find help. Search the achives of the gramps-users mailing list first, and if you cannot find the answer, send your question to firstname.lastname@example.org.
Finally, if there is a particular point that you feel should be documented (or where the existing documentation should be improved) then please look at How you can help. Perhaps you can write a tutorial on a specfic aspect, or improve this wiki, or help with the User Manual.