A cracked vent line can and probably will let sewage smells into the house.
Does the toilet in the GB rock at all? If it does, it is likely that the wax ring is compromised. The wax doesn't spring back, so as it rocks, it will get mashed down and leave a gap.
Is the toilet caulked around the bottom to the floor? If it isn't, a little dribble here and there can build up under the edge and smell pretty bad.
Is the ceiling under the GB open, or finished? If unfinished, can you see any moisture? That won't tell you if you are leaking gases, though.