Hi
If there's no pain or any other associated problem, this is most likely natural pigmentation (melanosis) which may occur on its own (physiological) or due to other factors such as smoking (smoker's melanosis). If the appearance of it is bothering you, you may get a depigmentation procedure done on your affected gums.
Next Steps
Visit a dentist and get a gum depigmentation treatment.
Health Tips
It is not a permanent treatment. Results last upto a year or so. After that it may have to be repeated.