The best user documentation for Gammu comes in form with man pages within the program. You can also view them on this site. Alternatively some information is available on the wiki, but it contains outdated content in many cases.
For checking support of some phone model, the best source is Gammu Phone Database which contains user experiences with Gammu and different phone models.
If you still have some questions, please check Frequently Asked Questions before asking on mailing list.
The developer documentation is mostly autogenerated or included in the sources, you can find the basic information and links on separate page.
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.
[GW]ammu