Take my advice with a grain of salt!
I used to work in a reptile store and I have kept reptiles forever. I've done some amateur herpetological "vet" work for folks that have had sick animals. Now, having said that, Amphibians are different from reptiles, but I've worked with them as well, just much more limited than my normal reptile (mostly snake) work. My first question is:
Are you sure this is gravel? See, sometimes animals can get a cut in their mouth and get an infection. Or, sometimes they get a fungus. Snakes and lizards can get mouth rot. It's a symptom most times and not the main problem itself. It is usually indicative of systemic infection, not just a mouth infection. It can cause lack of eating. I am not certain an aquatic frog can get it, but seeing as how herps in general can sometimes have similar ailments, it may be worth asking a qualified vet.
My Second question is: Did this appear overnight? Was there a small cut before this developed? Was there ANYTHING present before this happened? Lethargy, change in diet... anything new? Treatment for fungus and bacterial infection would differ. Sad thing is, most vets are mammal specialists. You might search the internet for a Herpetological society in your area and see if they have message boards or could recommend a local vet. I would be hesitant to tweeze anything I wasn't 100% sure was a foreign object. It would suck to rupture an infections boil.
Good luck.