I got mail from the Tiefbauamt, they say their internals (jährliche Gewässerunterhaltung) have it at Güllenbach, but they also refered me to the double name as it is present in wikipedia or other media.

I had hoped for a more indepth answer but I suppose that's as official as it gets for now. 
I think keeping it as Güllenbach with Gillenbach as alt name sounds good then. The Güll- -> Gill change seems pretty typical swabian to me but I guess it has not fully caught on so to say.

Yes, I am local, and I only ever knew it as Gillenbach from locals, hence the whole thing. But this seems like a good compromise to me aswell.