I'm the creator and main developer, and made the very first version by myself, the project is open source though so anyone can volunteer to improve it. There's been 55 contributors to the project so far! For example Pau who among other things wrote the entire music editor, and Toxa who was instrumental in the engine rewrite introducing GBVM that happened in v3.0.0.
If you ever want to help improve the application the code is all available at https://github.com/chrismaltby/gb-studio