Oh wow I didn't know that, interesting. But given that GMM owns ONEHD I wonder why it isn't their logo on everything...
GMMTV is a production company, owned by GMM Grammy. To put it simply, GMM Grammy is the big media conglomerate that owns a lot of these media-related business including music, TV, film, magazines etc.
(like, 70% of it).
When people mention GMM, they usually mean GMM Grammy the big umbrella.
Examples of those that GMM Grammy owns:
TV channels - OneHD, GMM25
Music labels - err, tons of companies
Production company - Exact, GMMTV, GDH (the latest sub I think they have is GMM Bravo)
As for the production companies, Exact usually only produces lakorns for OneHD.
GMMTV is pretty new, like a few years old, and produces lakorns for both GMM25 and OneHD (their currently airing boys-love series, SOTUS is aired on OneHD and the upcoming March/Baifern lakorn will also air on OneHD). And GMMTV officially subs their lakorns.
GDH in the meantime focuses on films but they also produce lakorns for both GMM25 and OneHD (for example Hormones was produced by GDH and aired on OneHD).
Both GMM25 and OneHD (sometimes) also produce lakorns in-house without using the production companies, like the Club Friday Series.
So in a nutshell, OneHD, GMM25, Exact, GMMTV and GDH are all owned by GMM Grammy.