Maybe a little late reaction :)
I think Gassen isn't a shortening of your name. I think Gassenhuber is a combination of two different names.
Gassen is an old German word, meaning alley. while Huber means : Man who owns a hube (a 30-60 acre parcel of land), from the Middle High German "huober" or "huob(e)ner." So maybe your grandfathers ancesters were more improtant Gassens, so they added the huber??? So I think it's just the other way around.
btw, my name is: Van Gassen and I am from The Netherlands