In housing co-ops, guff means something is communal property. This word allegedly began in the ICC co-ops in Ann Arbor, Michigan--in Mich House, specifically, and is typically considered an acronym for "Generally Unrestricted Free Food" (in Ann Arbor) or "General Use Food and Furniture" (in Austin). (Urban Dictionary says "General Unspecified Free Food", which may be an Ann Arbor thing as well) It seems to have spread to most housing co-ops in North America, though I have heard of one deviant co-op using the term "Blammo".
Co-ops are communal living spaces with shared, communal food, but you might also want to buy your own fancy cheeses that the co-op doesn't want to splurge for. As such, it's important to distinguish between communal and non-communal property. "Guff" is the distinguishing term.
Guff is not limited to food, however. As noted in the Austin backronym, it could be furniture, or most anything else, really. A house might have a guff bicycle, that anyone may ride--it's worth noting, however, that in this case you can't take it and keep it as your own. The desk a former member dumped in the shed, however, is something you can take and keep in your room as your own. It can be ambiguous. (Food, conveniently, is consumed, so it lacks any confusion about whether it's "take-and-keep-for-yourself" or "use-but-don't-keep-for-yourself".)
If you find yourself living in a co-op for more than a semester or two, you will find it hard to restrain from using this term in conversation outside the co-op scene, because it's just so darn useful. "Is that cake in the staff lounge guff?" is a question I feel the need to ask, well, every time there's non-obviously-guff cake in the staff lounge. "Up for grabs" just doesn't have the same flow.