I haven't been able to find any clear info about GMST's and when to clean them from a mod.
More than you might ever want to know about GMTS:
http://www.wikihost.org/w/mlox/evil_gmsts/.
http://www.mwmythicmods.com/argent/tech/gmst.html.
http://www.uesp.net/wiki/Tes3Mod:GMST_Contamination.
From what I have managed to gather is that they only show up if you make a mod not dependent on Tribunal and Bloodmoon.
That's the most common case, but I've seen some plugins that contain hundreds of GMSTs, obviously not entered by the author.
Is this correct, and if so does that mean that I don't need to clean my mods of GMST's if they are dependent on the two expansions?
You should always check. I've seen plenty of plugins on PES which the author claims are GMST free, but are not.