I am getting sunken spots in my leg muscles. Am I getting gangrene?

No, but Dr can check. Gangrene is dead flesh, which would cause not just a "sunken spot" but lots of other symptoms such as pain, swelling, redness, fever, etc..., if it were in or on one's leg muscles. However, "sunken spots" are unusual and should be examined by a primary care doctor.
No. That does not sound like the typical presentation of gangrene. You should see a physician.