As Stan said, the idea is that you use gitlab directly. Just make a Merge Request, you can do this online via the web editor or using git or using the gitlab app (I think).
As for 'What standards are used to determine if a mod is suitable?', the idea is that the community will self-manage, but so far only the 0 A.D. teams are maintainers so we would merge things. Ideally people will react to changes here or on gitlab directly. At the moment, I'll probably merge simple changes that are widely agreed upon, as for A27 essentially.