The best user documentation for Gammu comes in The Gammu Manual. It covers all aspects from installing Gammu, configuring it, running the programs up to SMSD documentation and C or Python API.
You can also download documentation in PDF:
For checking support of some phone model, the best source is Gammu Phone Database which contains user experiences with Gammu and different phone models.
As a developer, you might be also interested what is planned to happen in next Gammu and Wammu releases, you can find this information in the roadmap.
Additional hints about protocols can be found in
Gammu sources or you might want to use git repository with various GSM documentation.