G&P does have a huge license with magpul, but they aren't the only company that puts magpul parts on stock AEGs.
And from tons of experience working on them, I've decided that all PGC style LMG mechboxes are just crap. They're plagued with TM spec gearset fitment issues, some have proprietary tappet plates, and they're all used in conjunction with a CA style M249 hop chamber that was never very good to begin with, and always develops a horrible air leak later on. Not to mention box mags, can't speak for any others, but the CA boxes are total crap. I hear A&K is the way to go for boxes but I could be wrong...
The G&P 249 series V2 mechbox is definitely the way to go. I've never had any sealing, hopup or mechbox issue with my V2 LMG. And now that ARS makes sorbo cyl heads, you never have to worry about the mechbox cracking either. The box mags are solid, pretty sure the boxes are real steel plastic. And it uses a tried tested and awesome AUG hopup chamber.