Can you get a photo of the wound? "Gash" reads as a fairly severe wound. How did the wound occur?
The best thing you can do is clean the wound and provide good supportive care so her body can do the healing. You'll need to keep her isolated so that no other bird picks at it, because open wounds are like big flashing targets for the beaks of other chickens. Once the wound has been cleaned you will want to monitor for infection. You can use antibiotic cream (same stuff you get for people), just don't use anything with any sort of "caine" in it (ie lidocaine). Providing her a clean, protected area to recover in will also help to prevent insect activity on the wound.
young roosters are unpredictable, I would try putting them in isolation for a few days see if that don't give them an attitude adjustment. and see how that goes. even my pullets don't usually take more than once in chicken jail and they behave better. Also depends if you want to spend the time to modify their behavior or not. thats a decision you will have to make, but if they did it to one without any consequence they will most likely do it to others.
I wouldn't give them more than one if they are that aggressive