So I also do use gitlab quite a bit, but not as much recently. I went to compare. Gitlab actually does have a similiar ux experience, though I'd give the Github one just a bit of an edge. It looks like the key difference is that github converts a pasted image to an html image tag, while gitlab uses markdown with the width/height brackets that the end.

Honestly, I think using an html image tag is the right way to go. I type in markdown all the time, and I have no problem making links. But markdown image syntax I have to double check each time or let the editor figure it out. HTML image tags, I find easier to remember and read than a markdown one. (But maybe that's because I learned HTML before markdown).

Sounds like powerful UX others could also supports. Its all web stanards.