I'm only 19, and I became sexually active with my boyfriend in the past year, and I have had Gardnerella for months now.
My Gynocologist told me that some women struggle with Gardnerella and sometimes there's just not a whole lot you can do.
There are many factors that can cause your Gardnerealla or any type of Bacterial Vaginosis.
One factor may be sperm. If you are on a type of birth control, and you're not using condoms, sperm can really mess up the natural environment of your vagina because sperm is a BASE, it's not acidic.
After menstration, your vagina is more vulnerable to developing bacterial vaginosis because it is in a more base-like state.
You should talk to you gyno and look into getting a Vinegar Douche. It's just vinegar and water, and it can help return the bacteria in your vagina to an acidic state.
Other than sperm... there are other factors. Look at the lubricants you may be using. And really watch your hygiene. Don't use scented soaps often... and my gyno has also suggested eating yogurt (like Yoplait) which contains live cultures which will also help maintain a balance of bacteria!
Hope that helps... it's amazing how much you learn on the internet.