What can I do to lighten the color of my gums?

Nothing that I know of.
Are your gums dark because they are unhealthy?

Some teeth whitening products will bleach the gums but this is definitely not something I'd advise because that's not the intended use of the product.

Some people just have naturally dark gums just like some people have darker skin tones. Focus on dental health first.

The following has been posted more than once by Simonellis1964 and may answer your question:

I agree its very important to see a periodontist --this type of specialist is an expert in gums and gingival tissue. They can help treat your dark or discolored gums and also assess the health of your gums... most dentists don't treat dark gums....

Because I am african american I had a similar problem of excess melanin in my gum tissue. I looked extensively for someone that specializes in treating pigmented, dark gums .... I wrote about this in another post but to summarize -- I had my gums treated by Dr Farnoosh in Los Angeles, California. The procedure is called "gum bleaching" its something you might want to look into...

This really helped me feel more comfortable smiling.... Dr. Farnoosh basically invented this procedure that changes your gums from dark or discolored to healthy pink. I was amazed when I heard that this was possible. When I talked to my dentist before travelling to Los Angeles, my dentist said that Dr. Farnoosh is famous for this procedure so I should go to him. I travelled to LA and I liked the Doctor alot he was very compassionate and the procedure was not very uncomfortable.

His website is below -- do your research and good luck. You will be amazed by how changing the color of your gums will make you more confident -- I was and am very happy with the results.....

http://www.thetotalsmile.com/gums/dark-gums.cfm

Let me know if you have any other questions.