The open source game engine that is used to create the application.
The open source godot MIDI player godot addon by that is used to play notes through soundfont files.
Free software to edit soundfont files. It was used to seperate instruments from .sf2 files with multiple instruments were included.
This can be achieved in the export with checkboxes.
Free and open source DAW. It can be used to combine / rearrange / repeat .wav files made with gamepad piano to a new composition.
Websites where soundfonts were found for free.
Udemy course that provided music insights by an excellent teacher that were used for creating Gamepad Piano.