The Game System License has a what appears to be a big hole in it.
Certain monsters, such as the Beholder and the Displacer Beast, (and possibly other non-monster things) are excluded from the SRD.
Before 4e, these things were considered Product Identity (despite the fact that D&D’s claim on the displacer beast is tenuous) and were not included in the 3.x SRD or as Open Gaming Content.
The thing is, as far as I can tell, the GSL only limits what you can do with respect to terms in the SRD. As such, I don’t think there is anything in the GSL stopping someone from using, say, beholders in a product – or redefining them in a way that wouldn’t be permitted of a Bodak or Bulette (both of which are in the SRD).
Oddly, the trademark notice in the Monster Manual doesn’t claim a trademark on any monster names or likenesses, either… so that doesn’t appear to be a restriction.
Of course, if you published under the GSL and abused this, I believe that Wizards could probably just alter the license retroactively in order to stop you from publishing your book in which beholders are happy, purple, smiling beasties. It is rather one-sided.
This should not be construed as legal advice, by the way.
(Yes, I have a law degree. Pretend I’m not using it. )