DC Comics Presents 50, IIRC, though it sounds as if you've found it already.
One further point -- The schism between the Guardians and the Controllers occurred after the Green Lantern Corps was first formed, according to the retcon which introduced the idea. So the proto-Controllers would have been a part of the creation of the Green Lanterns in the first place, ie had an equal claim to having 'created' it.
Seems that they went quite a bit further with it than did the Guardians, given the state of Oa and the Corps in the 30th century.