It is "gubbe" in sing. indefinite form, and "gubben" in definite. In plural it is gubber/gubbene.
The word means basically "old man", and may be used either derogatively (old, grumpy/fossilized chap), or be a little rough term of endearment about ones husband or father (my old man). Used only about the man, not to adress him.
I've lived all over Norway and heard many dialects, and I can't remember hearing 'guben' rather than 'gubben', but who knows? I found these two sentences on a Google search, but they may just be typos: (under the heading 'drita full gubbe') 'ca 1 time våkner jeg opp og ser at guben står vendt mot komoden' and (separately) 'Så du ville vært villig til å satse på at guben dopa seg litt i helga'.