There was already a limited edition GTS for the previous generation (or the one before) if I remember well. Only for the M3.
It is more a marketting exercize, the "fun" to develop an even sportier model for a few, as a showcase, but not really the will to introduce an new "familly".
Here it is meant to introduce their new "water injection" thing.
But you know how things evolve, they can decide tomorrow to develop the GTS versions to other models as well in the future.
That would be cool

Cheers, Mark