I think you need to get rid of the gym control bonus for this, but I definitely do agree that awarding long-term commitment to the gym would be a good thing to have. I would go with +1 for silver, +2 for gold, bronze is so easy to get it doesn't need a reward.
The reward of extra items is pretty good honestly. I have a cluster of 3 gyms near my home that can be looped around in 5 minutes. The loop produces 6/9 items(depending on color) from the gyms with no badge and 15/21 with gold badge. In an hour that makes 72/108 vs 180/252. Huge difference for grinding efficiency.