None that is publicly available that I'm aware of, unfortunately.
No real tooling either other than whatever you choose to use for the build process. For the projects I've done this for that ends up being a very simple Makefile which does a 'go run' on the code generation engine code prior to a 'go build' of the resulting combined package of generated & hand-written code.
Custom -- most of the code generation I've done has been about binding Go functionality into UI markup languages (similar to the way ActionScript binding in Flex works, for those who have used that) as opposed to generics-like functionality.
I've never used gen, it looks pretty nice though, at least based on the linked docs and it is probably a good answer to Pxtl's question about publicly available examples.
No real tooling either other than whatever you choose to use for the build process. For the projects I've done this for that ends up being a very simple Makefile which does a 'go run' on the code generation engine code prior to a 'go build' of the resulting combined package of generated & hand-written code.