It depends on the type of gag you want. Do you want a legitimate ball, such as silicone, ball with holes (like a whiffle ball), or something like a bar? Personally I prefer long gags, such as the bit gag or ball gag offered here! If those bother her, then there are still a number of options to choose from. Basically, anything that can fit in her mouth can be a gag, whether it be a washcloth, her panties, or a scarf! Just make sure that it's clean and doesn't have anything that could potentially harm or hurt her.

Also, because the verbal communication will be hindered, make sure that you have some sort of non-verbal safeword in place. Be safe, sane, and consensual.