At this time, there is no GCP managed option that would allow you to serve your packages via a custom domain.
However, there is an existing
feature request that the Artifact Registry (AR) team is studying. I recommend starring (The more stars, the more visibility the request gets) and CC'ing yourself to the thread that way you will get updates regarding the request.
In the meantime, as mentioned in this
Stack Overflow post, the only workaround available at the moment is to build a "reverse proxy" on top of your AR to forward incoming requests while still serving the traffic on the custom domain name you own. More details on this can be found in the original
blog article.
Note that the above workaround is only available for Docker, and it requires the registry to be publicly accessible. If either of these requirements is not feasible for you at this time, then it would be best to wait for a response from the AR team in regarding the feature request.