Mar-D on Gökdeniz is actually optional, and intended for standalone use of Gökdeniz (on small vessel lacking a 3D search radar and a mast to support it)
On Barbaros there are two CIWS, this could be reason why Mar-D was not attached to Gökdeniz.
On I-Class, it will make sense to wait and see whether it was moved on a mast for better coverage. They might have changed the design.
Or maybe it will be adequate to be position it on Gökdeniz since there is one CIWS and it wouldn't matter if Mar-D can specifically scan blind spots of the CIWS (which is also covered by Smart-S).
So from that point of view it is standardized, Barbaros had 2 masts which made it easier. and we haven't seen I-Class fully yet.
What you say is true, there are 2 CIWS on the MEKO, but I think there are also 2 CIWS on the I-class.
One of the main differences between the I-class and the Meko 200 is the main 76mm cannon.
As far as I know I-class is to get the super Rapid 76mm with possible the dart / strales munition (so the main cannon will function as CIWS)
The Meko 200 Barbaros is to keep her main cannon which is a 127mm cannon (this one cannot function as CIWS), that's IMO why Barbaros got Gökdeniz CIWS on the front.
So the MAR-D is actually part of the Gökdeniz target acquisition / tracking. But I have still no satisfactory answer on why the Barbaros has a second mast with MAR-D placed high and with I-class this is not the case.
Second strange issue I find the choice of Phalanx at the rear of the ship, why not a Gökdeniz there too?