I'm not entirely sure if Testbed is still open, but if it is, you can follow
this link to apply to be added.
One of the best places to actually start, is to learn JavaScript because of the similarities in logic and syntax with GS2. It's not required but it can definitely give you a jump start.
For function lists, you can go
here, as well as
here (which is what I personally use cause it's easy to search for whatever I'm looking for).
Most development info is readily available with a quick Google search (ie on websites like the GraalOnline forums in the scripting archive section and GraalOnline.net). However, it is poorly documented which is one of the major issues of GS2.