You’ll need to install gforth to be able to run gbforth.
On mac, you can use
brew install gforth
Alternatively, you can run gbforth in a Docker container. This does not require you to have any other dependencies (apart from Docker) installed. Use the amshackers/gbforth docker image:
docker run amshackers/gbforth
To compile your project, mount your project directory as a volume and pass the input and output file as CLI arguments:
docker run --rm -v "$PWD":/data amshackers/gbforth game.fs game.gb