Years ago, I came across this thread (NSFW). I liked the combination gag someone made, and printed it. Since then, I mostly forgot about it, until today. I decided to start it from scratch, copying mostly from user SissyTina's design. I fixed some issues, and made it really nice. The SolidWorks 2016 file is fully parametric and variable driven, too.
I would recommend using medium to large metal rings through the holes in the bit gag, in order to avoid the tongue rotating the gag in the mouth.