re.DOTALL
does nothing in this case so I've just taken it out for simplicity's sake:
>>> import re
>>> max(re.findall("g+", "fggfggggfggfg"), key=len)
'gggg'
If you need all of them in order of length:
>>> sorted(re.findall("g+", "fggfggggfggfg"), key=len, reverse=True)
['gggg', 'gg', 'gg', 'g']