The gas problem of the Bullmastiff ain't THAT bad from my experience, my boxer have much worse gas problems than the old Bullmastiff I used to have.
They are great dogs, and require very little grooming, but here are some things you should know about this breed before you buy:
1) They slobber, and I mean, THEY SLOBBER, as in you have to keep a rag lying around in each room to wipe their saliva.
2) Some gas, not as bad as Boxers though.
3) They snore, quite loudly, which could be a nightmare for you if you're a light sleeper.
4) Bullmastiffs would usually get up every 2 hours, take a walk around the house, and make sure everything is ok, if they find a toy however, it's party time at 2am!
5) They aren't Labradors or Border Collies, they aren't exactly the easiest dogs to train.
6) They can be quite dominant, only get one if you are capable of maintaining a pack leader status.
7) They are very low energy dogs, too much exercise can hurt them, if you want a more active dog, get a Dogue De Bordeaux.
8) They can be quite protective, and needs plenty of socialization.
9) This is quite obvious, but should be said for the record: They can be quite costy, they're massive dogs, thus requiring more food, and larger vaccination quantities, which translate to more money from your pocket.