You should try swapping which cat is on which side of the gate so they get used to each other's smell being all over the house.
You can also try Feliway the cat appeasing pheromone to drop the tension between them. Its available at pet supply stores and online. It works to reduce anxiety using a cat's normal 'friendly' pheromone from the chin area.
Make sure they are not just play fighting - my cats do that all the time and then sleep together after a round of boxing.
Hope this helps you!
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