My first recommendation to you is that you go ahead and see a vet. A vet has prescription products that will take care of all stages of mites, eliminating a lot of trouble for you. These links will take you to directories of vets who treat small rodents:
However, since you asked for things to try at home first, I'll give you the recommendations of the American Gerbil Society. They recommend a product made for small rodents that contains pyrethrins. If your local pet store doesn't carry one, here is an online source:
You'll need to completely empty the cage. Clean it with bleach and soap. All toys, dishes, and the outside of the water bottle also need to be cleaned this way. Rinse everything well, dry it, and then spray with the recommended flea/mite spray. Put in brand-new bedding and spray it. Then spray the gerbil and your hands. Work the spray completely down to the gerbil's skin. You can read about the process in detail on the AGS site:
Here's the part that makes it difficult. You have to check for mites every day, and each time you find even one, repeat the entire process. The AGS site describes the best way to detect additional mites. Old bedding must be bagged and sealed tightly. You'll have to be diligent about this, and can't give up if you hope to eliminate the mites.
It's possible that repeated spraying of the gerbil and the cage will eventually lead to side-effects, which is another reason I prefer veterinary treatment from the beginning. However, you are most familiar with your gerbil and your situation, and so are in the best position to make a decision about what to do. If you have more questions, just let me know by clicking on REPLY. I hope that whatever you decide, you'll be able to eliminate the little pests completely.