The face plate onto which the handle is attached says Gerber....the designs of the handle and face plate are the same. I ordered the unit about 8-9 years ago from a plumbing supply house and it has never been touched. Do you still think that they put in a Moen cartridge?
I was able to get the handle off....I attach a picture of the cartridge which rotates up and down to vary pressure....I have not been able to locate anything on the web that indicates how I should go about this.
Of course you would turn the water off first. Don't attempt this until you have a replacement. Go to a plumbing supply house that sells Gerber for parts they can confirm my recollection that there is just one style Gerber cartridge of this vintage