You can submit package definitions to Guix by sending pull requests to the guix-devel mailing list.
The process looks like this:

Fork the guix git repository
Add your package definition in a branch
Send a pull request for your branch to guix-devel
Discuss the change with reviewers and address any feedback
Once your change is approved, it will be merged into master

So in more detail:

Fork the repo on GitHub (or GitLab if you prefer)
Commit your package definition in a separate branch
Use git send-email or format-patch/git-format-patch and email to send a pull request to guix-devel
Subscribe to guix-devel and participate in the review process
Address any feedback or comments
Once approved, a Guix maintainer will merge your branch

The Guix manual has a more detailed walkthrough of this process.
Hope that helps!
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