The issue with GitLab's UX right now, is there is a lot of little things, which adds up to big things. This is expected since UX is hard and if you look at GitHub's interface from 5 years ago, they (GitHub) would probably feel embarrassed by some of the things that they have done.
For things to improve, you need a lot of feedback/iterations and I won't be surprised if GitHub's UX evolution was the result of strong feedback from designers outside of GitHub. By being the first movers, GitHub was able to attract everybody, which included designers.
GitLab right now is not attractive for designers. To overcome this, I would personally put some resources into showcasing how you can tweak GitLab's CSS without having to dig into Ruby or JavaScript or anything else. And if it is not easy to do, design GitLab so it is.
Right now the fonts in GitLab really bug me, and I want to use the font that GitHub is using. GitHub has really nailed fonts among other things.
If there was a simple way for people to tweak GitLab's UX without having to understand anything else, I can see it drawing in a lot more people and creating a lot more feedback. Also create a way for people to showcase their tweaks. I'm sure 90% of them will be crap, but that 10% can make all the difference in the world.
I might be over analyzing things, but I think GitLab's desire to be no longer called a "GitHub clone" is creating too many poor design decisions. The recent move to put the search icon in the middle of the search text field is an example of this. I'm pretty sure designers at Google, Twitter, Microsoft, etc. played around with the icon placement and found that putting it in the middle was not a good idea. If it was a good idea, we would have seen more sites/products adopt this.
So to make the long story short, to improve your UX situation, make it insanely easy for designers to tweak things and to share/showcase their tweaks.