Recently, I set up my own Gentoo overlay on GitLab with CI using their shared GitLab Runner. I had a lot of trouble at first with getting the CI to cooperate with what I wanted. Getting repoman to work in the docker image was pretty irritating until I found a docker image specifically for testing using repoman. Once I found that, it was super easy!
My main purpose for setting up an overlay was primarily to learn how it all works. A friend and I had done it before but I wasn’t quite so familiar with the actual setup. Secondary reason was the desire to use a more recent version of atheme-services as it appears no longer maintained in portage but was recently revived upstream. I have a few other ideas for what I may want to put in there, but for now it is just small time modifications to ebuilds.