Interesting topic.
Gillenbach is even on wikipedia 
https://de.wikipedia.org/wiki/Gillenbach_(Schussen) with quite some information about it.

Gillenbach seems closer to the original name or meaning, and maybe more popular.

Güllenbach (Gülle = manure/slurry) in local dialect would be pronounced almost the same, and could be a "re-translation" from Gillenbach to "standard German".

I'm curious to see what Ravensburg answers.

BTW: we can communicate in German if you prefer - you seem to be a local.