N.D.
I admit, I did laugh when I first read your e-mail but in all seriousness, if you can't handle it, it's time to wake up Dad and have him comfort Jr. Sometimes there is nothing you can do to curb it and so you can either wake up Daddy or wait outside the bathroom for him.
By the way, I don't know what is causing you to gag...the noise or the smell but IF it's the smell, you might try (and this is going to sound weird) but putting a small rolled up piece of mint gum in each nostril helps. Long story short, I use to work in a hospital and I had to bathe some unsavory smelling people at times and this did the trick for me.
Good luck.
N