The only official documentation available for the Gambas development environment is the documentation on the Internet.
For Gambas version 3, there is a new official Gambas wiki at http://gambaswiki.org/wiki.
Searching works easiest with Google search using this search string for example for information on the definition RichText or on Paint:
Figure 1.1.2.1: The new Gambas documentation.
Under the link http://gambasdoc.org/help?en you will find the old documentation for version 2, which is no longer maintained. You can switch directly to the documentation for version 3 (→ red marked area):
Figure 1.1.2.2: Gambas documentation (read-only)
You will get help for the following areas, among others:
Unfortunately, there are only a few passages in the documentation for which there is also a German translation.
At http://gambas.sourceforge.net/en/main.html# (./Help/Books) you can find also online books for Gambas 3:
Figure 1.1.2.3: Online book in german
An English translation of the German Gambas-Buch introduced above. Start reading the English version at https://gambas-buch.de/dwen/doku.php.
Figure 1.1.2.3: Online book in english
Very informative is the overview on the start page of http://gambas.sourceforge.net/en/main.html on Gambas projects, compiled from different fields of application. Often the developers not only make the programme available under a stated licence, but also the complete Gambas project - including all source code. The authors have also made a project on 'Data Type Struct' available for download: