Your assessment is right: the grumpc compiler takes a single Python file and spits out a Go package. Incidentally, this means you can import a Python module into Go code pretty easily.
I don't have a ready solution for building a large existing project but I'll write up a quick doc to outline the process. The trickiest bit is that the Python statement "import foo.bar" translates to a Go import: import "prefix/foo/bar". Currently prefix always points at the grumpc/lib directory so that's one way to integrate your code, but I need to make it more configurable.
I question the transpiler. I think I'd much rather prefer a solution like Jython.
Edited to add: The difference is that Jython doesn't covert python to JVM bytecode.
One advantage of an interpreter in general is that one important use case for Python is interactive scripting, as data scientists do.
Just tried this out on a reasonably complex project to see what it outputs. Looks like it only handles individual files and not any python imports in those files. So for now you have to manually convert each file in the project and put them into the correct location within the build/src/grumpy/lib directory to get your dependencies imported. Unless I missed something somewhere.. The documentation is a bit sparse.
Overall I think the project has a lot of potential and I'm hoping it continues to be actively developed to smooth out some of the rough edges.