Is it true that gargling with salt water helps with tonsil stones?

Yes: Tonsil "stones" are concretions of microscopic food particles, sloughing mucosa cells from the lining of the oral cavity, and bacteria (usually in the form of "biofilm"). Anything that causes them to be dislodged will help keep those pits in the tonsil clear. I have even known some patients who use a "water pik" to keep their tonsils clear of stones.

Sometimes: Pockets in the tonsils, also called crypts, can catch food particles and then the minerals in your saliva can and will coat them to turn them into tonsiliths. If you gargle after eating, they may be fewer and easier to clear.