well the problem with both betta and gourami is that they both have very unpredictable personalities
both gourami and betta can be friendly, happy, and even get along in some community tanks, while others are nippy and love to chase, then there are the ones that no matter how big or planted your tank is, they are determined to get the "intruder away from their territory"
the last one i have described, beleive it or not, can also be true for gourami
yes, the gourami would get bigger, but bettas are capable of bullying and chasing them until they are weak and diseased from stress
some people have managed to balance these two together but like i said, they come in ALL personalities
and you do NOT know until you put them in with the specific fish in mind
you could see a betta in a community tank in the store and put it into your gourami tank at home and watch them kill each other
IMO this should not be done
one should not mix with the other UNLESS you spent the next WEEK (they don't act themselves immediately as they have to get comfortable in the new sorroundings) have someone check on the tank hourly to make sure noone is fighting