Ha, you talk about being laconic! My first thought is to use template metrics to figure out why the site takes so long to build locally, but maybe you’ve got your own thing going with some massive amount of data or something…
If you aren’t looking for advice reducing build time, I’d look at the issue as being the GitLab’s hardware not liking how long it is taking to clean up your project. If it is using the gitlab.org runners, it is possible your jobs are sharing capacity with a bunch of projects, and that could time it out. I’d try a dedicated runner, a different way to put the site files into the repo served as your site…
Also, how about removing --gc? Move whatever that cleans into a different part of the build chain. 
Check out Requesting Help and post repos if you want more eyes to look at your issue. 