I agree with everyone that that is too much to treat with just topical. I highly recommend trying to get an antibiotic into her system.
Pennicillin G as previously mentioned is available at most feed stores. In Ontario it cost $6 for a small bottle the last time I bought it, both from the feed store and from the large animal farm vet. It is available without prescription, so if your vet office has it in stock, the desk clerk can just sell it to you. Large animal Vets will also have the erythromycin/gallimycin powder, it is more like $17, though you don't have to use needles. 28 gauge needles are at the Shoppers Drug Mart in a ten pack for diabetics for a little over two bucks.
It can get infected under the scab with an anaerobic bacteria like Staph.
Is her health otherwise seeming totally fine, appetite, energy, etc.? How did she get that gash, your rooster?