There may be an underlying health issue causing the odor and gas such as irritable bowel or parasites (giardia). It is always best to have your vet perform whatever tests are neceassary to rule out a medical reason. If you have done this already, there are a few suggestions for you to try:
1) Add a teaspoon of PLAIN yogurt to your cat's food - or separately- it doesn't matter which way. Plain yogurt has 'good' bacteria in it that will help balance her intestinal/tummy area.
2) Adding fiber to the diet actually may help even though it sounds like a 'backwards' suggestion. Mix Metamucil (1/8 tsp) with water and add to food.
3) It could be a particular ingredient in the food (and the same ingredient may actually have been in the previous food too) that causes this problem. The best way to find this out is to switch her diet to a allergen free food - a prescription diet available at your vet's office. The food is a bit expensive however a trial run of 4-6 weeks could easily prove to be the solution - and from what it sounds like, probably worth it to you!
4) If she's a fast eater - place a canned good in the middle of her food dish so she has to eat around it. Many times gulping food down is the reason for gas in pets.
I truly hope this gives you options you have not tried in the past and you discover that one of them will provide the answer you have been waiting for!
Please reply to any questions or let me know if you need further assistance!Otherwise, I hope you have found this answer satsifactory and worth acceptance!
Oliver's Mom