Alex Samorukov commented:
Just in case - if there is any GNOME supported CI which allows MacOS builds and is integrated with gitlab - i am happy to use it. Currently full build with all dependencies (and without cache) takes ~3-4 hours, so build system should allow that.
As @Jehan already mentioned - Apple does not allow to do build on non apple hardware as well as to do cross-compilation (what is technically doable). Folks for CircleCI upgraded my account for free when i asked to do that, as before i was building images on my own laptop. All last GIMP/MacOS builds were done using CircleCI, so it works really well. As for now only my intention is to move everything from my personal Github repo to the Gitlab, so there will be less dependencies and more trust