Good question! There is no explicit OWNERS file (as there's no tooling to use such a file), but we should use the same OWNERS as are currently in chromium tools/gn/OWNERS for now. Those are: brettw, dpranke, sdefresne. (Maybe we should re-add an OWNERS file for clarity?)
The high-level goal wasn't really to change anything about GN's development, rather only to physically move the code, and avoid the need to pull the entirety of the Chromium repository to build/hack on GN. So if in doubt, do the same thing as when the code lived in Chromium. :)