M.R.
Quit paying attention to it. Seriously.
She has learned that when she sticks her fingers down her throat it makes this really neat and goofy sensation. Her thought process probably goes something like this: "Wow, what's this? I can make myself make funny noises and feelings when I stick my finger in my throat. Let's try it again! Whee neat - I never knew I had this skill. Whoa, wait what's this? Mommy's making a big deal about this? It gets her attention? Hmmm let's do it again and again! This is neat fun!"
My firstborn did this. Grossed me out. I ignored it and the phase quickly passed. It's totally normal and I doubt you have a budding bulimic on your hands. You do have a smart little girl that is experimenting with her body and learning more about how it works.