I did everything with GDScript for the rapid prototyping and it was very easy to get into after using Python. Both languages use more or less the same syntax. There was no reason to use C# or even C++/GDNative for performance at this point but both are valid options for later when it might become necessary.
I wouldn't generally mind code sharing or even participation. In fact I plan to make it public at some point so other developers can have an easy starting point with Godot. My current issue is, due to the rapid prototyping, the code is messy and tangled and I know that large (old) parts are redundant by now. One of the primary reasons to make the code public is to FORCE myself to refactor it beforehand and make each feature more modular. I am hesitant to share it in its current state.
If you are new to Godot I would suggest, since Unity and Godot have sometimes very different workflows, that you get yourself comfortable with how Godot handles nodes and scripts by working through the 'Step by Step' section and demos on the official docs. I know I did and it really helped me out and when you are done I might have finished refactoring by then so you can jump right into.