Cats have normal bacteria in their mouths, just like we do. When that bacteria gets into a wound, it grows and creates infection, no matter how healthy your cat is. Your gerbil will likely not show signs until it's too late.
Anti-bacterial soap = any soap that's without fragrance and labelled anti-bacterial or anti-microbial.
I would highly recommend you seek veterinary advice and an antibiotic. In addition, they may want to check the wound for subcutaneous (under the skin) pockets. When a cat bites down it usually pulls, creating a space between the skin and the muscle layers - this is perfect breeding ground for bacteria.