@Gina voted for GitLab mostly because I have direct experience with it and I have none with Forgejo. Also I could be wrong but it seems to be more mature.
It’s really a hard question though. One is backed by a company which could always turn. But it’s also a lot more likely to be around and actively developed in 15 years (IMO).
I’ve self hosted GitLab CE (and more recently upgraded to EE for one feature which I don’t remember) for over 10 years and have no complaints.