If you have a gag that has a plastic or rubber ball that is removable, you can replace it with these smooth, non-porous silicone balls that do not have a taste or smell. The hole diameter is large enough that rope could fit through as well.  These balls can be sterilized by boiling but plain soap and water cleans them just fine. 

Ball diameter: 1.75"

Hole diameter: 1/2"