I have a fairly new kitten, just spayed & when I bought her home from the vets I asked him about her bad breath. He looked into her mouth & said he thought it was gingivitis. He then said, that he couldn't cure it, as antibiotic treatment is temporary, & the problem returns.
Her teeth look beautiful but her gums were red. He said I could try cleaning her teeth. At worst the cat will lose her teeth. She is not a year old yet & I came away a bit shattered, I wasn't expecting that in so young a cat.
Any helpful advise please?

I'd get a second opinion. Feline gingivitis is often caused by an underlying disease. The cat could have feline stomatitis, a painful inflammation of the mouth, often caused by feline leukemia, feline AIDS, or calcivirus. Please take your kitty to another vet. I'm curious, where did you get your kitty? Please keep us posted and good luck...