If you want to put a Gemballa sticker on your car, then by all means put one on your car. I'm sure Gemballa wouldn't mind the free advertising.
I have Gemballa Competition wheels on mine and a Gemballa license plate frame. Does that make me a poser? Beats me. The guy I bought my wheels from had the Gemballa stickers on his doors like the Gemballa boxsters out at the time, yet he only had their wheels. Some gave him grief over it and others thought it looked cool. I've always been under the impression that if you buy something, you can do whatever you want to it.
I have my car serviced at RUF Autocentre and they have complained (jokingly, they still like my money) about the license plate frame advertising the competition, but haven't given me one of theirs to put on. And some of their tech's seem to like my wheels better than the RUF wheels.
I don't have an "S" model, but I have an "S" muffler, so I guess I'm a poser because I don't correct people who complement my Boxster S (even though you can tell it's not an S by looking at the front bumper and brakes).
Life's too short to give a dang what other folks think. If it's what you want, then you should have it.
I'll see if I can come up with at least a better pic of the sticker, if not the dimensions too. I know a guy that had some actual Gemballa stickers that he's not using.