I realise I'm a little late to the party in regards to GMC, that it's been out for quite a while now but that's becasue it's taken me a few attempts to wrap my head around why anyone would see this as a good or worthwhile update, or why White Wolf/Onyx Path/CCP would see this as the best way forward for nWoD which was fairly straight forward.
I've been playing WoD since classic, I'm aware that I'm in a minority who actually enjoy nWod (Pre GMC)It was a simple system, a lot cleaner than classic WoD in many ways without being bogged down by a metaplot that can cause issues at the table (I've seen them) between players and ST's. GMC adds a metaplot, it overly complicates some aspects of nWoD, and makes other things a bit to wishy washy to the point of being far more abusable, this however isn't what truely annoys me about GMC, metaplots can be ignored with a firmer stance at games, wishy washy rules can be house ruled with a bit more structure and unlikable rules can be adjusted. What annoys me is that many of the rules changes already existed in nWoD through various suppliment books, most notably Mirrors as optional rules, where ST's could use ones they liked and ditch ones they didn't.
Onyx Path however seem to be intent from what I've seen over the past year or so on forcing these once optional rules on all their new publications instead of just simply cleaning up or refreshing things to a basic level that doesn't fundamentally change how a almost perfectly good system worked (I'm being honest nWoD wasn't perfect just more so that I feel GMC is)
Summerising my general feelings briefly at the end here, GMC wasn't needed, or to be more accurate I would have probably been more accepting of it if it where presented as optional rule changes along the same lines as Mirrors, and if the metaplot was clearly marketed and just ideas and not the foundation.