Do you want to get these packages officially into Gentoo, or do you only need them for yourself? If you only need them for yourself then I would forget about Gentoo specific packaging, and just install everything under your own user. If you need it on multiple machines then automate it e.g. with Ansible.
If you really want to go the distribution-specific route then look into how Gentoo handles Java libraries. I know Debian has its own Maven repository on the file system where officially packages Java packages end up. IIRC it then packages a patched leiningen to make use of that.
It’s all far from trivial, since each dependency needs to be packaged first. Check out the episode of the REPL with Elana Hashman if you’re curious about the process.