Seperate her from the male (but move the male out, rather than her, or she'll be stressed unduly).
Are you definite the wound is getting bigger? Have you seen the male nipping at her more or does it appear to be getting larger on its own?
Melafix is a good idea. Salt can also help but make sure none of your other fish are intolerant of it (eg: cories).
Is there any sign of a secondary infection? Any white or fuzzy edges to the injury? If there is, can you say whether it's fungal or bacterial? Either way, most anti-fungus & finrot meds. will work to fight secondary infections but lowering the water temperature to 76 deg F is also crucial. Only do this if you can see there's a secondary infection as lowering the temp. also slows healing and can be stressful if not done very gradualy.
Any inflammation anywhere else on the body? How is her appetite? Is she bloated? Behaviour-wise, is she acting as usual or does she appear lethargic? The reason I'm asking these questions is incase this is an ulcer. If it is, and is getting larger, there is unfortunately little chance of recovery. A strong antibiotic, more effective if added to food, will be necessary and you'll also have to isolate. Fingers crossed this is not what she has.
Assuming she survives this experience, I suggest you consider buying another female in future so that the male's attention is split between the females. That way there's less chance of something like this happening again. Unfortunately, most gouramies shouldn't be kept as male/female pairs.