I think both are largely comparable, especially if you don’t use “advanced” features such as team work, code reviews or CI/CD, and you only want to host your source and use simple features such as issues.
If that matters to you, GitHub is owned by Microsoft whereas GitLab is open-source. With GitLab, you can download a full copy of your repo along with its metadata (issues, comments, merge requests and so on) for archival or to put it on your own hosted instance (if that becomes necessary to you, which I doubt).
GitHub is way more popular though, so you may have better visibility there, but I’m not sure that really matters for a small Adventuron project.
Disclaimer: I prefer GitLab, mainly because I favour open-source projects. Also, I used Bitbucket before, but it stopped supporting Mercurial and I had to convert all my repos to Git myself, and import them somewhere on GitLab, and it was difficult/impossible to keep metadata such as comments and issues. (Luckily I didn’t have a lot of metadata.) This wouldn’t have happened with an open-source platform such as GitLab.
Also, I’m far from being an expert and mainly use GitLab for small personnal projects.