Source code documentation
The Gekko source code is freely available (under the open-source GNU GPL licence), and it is the intention to keep the source code well-documented.
Since the source code documentation relates to Gekko 2.0, and the source code has changed quite a lot in Gekko 3.0, there is an unresolved project of documenting the Gekko 3.0 source code. Still, the source code documentation related to 2.0 still gives a good idea of a lot of the components.
Above, there is also a link to the upcoming API (Application programming interface). For now, only the TimeSeries class is documented (but still, this is an important class regarding a timeseries-oriented software like Gekko).