New Army "Caliber Configuration Study"

[woohoo]

Will the 6.5x43 Grendel GPC work in the magazine well of the M4 platform? And how does it compare to both the 6.5x39 and 6.5x45?

No, the OAL is 2.5" and it isn't a 6.5x43"Grendel" The Grendel is 6.5x39.
If it is based on the Carcano case like my 6.5x45 it will have 40gr capacity. The Grendel has 36gr and my 6.5x45 has 42 gr.
All of these things are being thrown out there as a GPC along with a 6.5 in a 6.8 case, none of the are...yet. Any of them could be but until the military adopts them as such they are just little known wildcats.