It's a slight struggle to maintain 77kg at 190cm. What have you been trying? I used to think nothing works, but they do say to keep a food (intake) and exercise journal so you hold yourself accountable.
I do have a small frame (thin wrists, smaller bones, etc) so I suspect my biology wants me to stay at like 75kg.
It looks like a curse due to current body ideals, but it’s probably a blessing.
If you can sustain that without gaining muscle or fat, I would suspect some sort of medical issue.
(I wouldn’t recommend GOMAD to most people. Even in your case, barbells + massive amounts of food would be my primary suggestion. However, I wouldn’t start immediately, as the whole gym/train/mer/physio system is mostly down or inadvisable from covid. You can hurt yourself with barbells)
The amount of happiness to be gained if only we could trade such character traits...