Unfortunately I was unable to find it. They provide a solution, it’s a ruby script that generates a pdf, but unfortunately (due to the very large number of objects) it’s unreadable.
What I did was to search in structure.sql the tables I was interested in and find their foreign and primary key constraints (they are also defined in structure.sql) and draw them manually in draw.io or smth similar, very tedious work.
Thank you @RaduDr for the answer! It’s a really pity that GitLab doesn’t provide full ERD or at least most useful tables relationships. Hope they at least thinking of this in the nearest future