i hate to break your heart but only rest can fix the problem once you have it
the key is NOT getting it in the first place.. proper shoes and not over training is going to be how.. i would run barefoot on grass in the first place.. people who run on hard surfaces are itching for problems
and as far as your question why do you need to quit the gym? there are plenty of lifts at the gym you can do even with plantar issues. it is running that you need to stop