Hey, relative beginner, had a few questions:
1) If I use pudding, syrup, cake batter or honey, will any/all of those be dissolvable to the point I can do it in the bathtub and hope to wash it down the drain without destroying the tub?

2) Has anybody ever done gunging with large amounts of liquid soap as the gunge? Found these, each is a couple of gallons of liquid soap for (relatively) cheap...
https://www.webstaurantstore.com/advantage-chemicals-1-gallon-foaming-hand-soap-4-case/146FOAMSOAP.html

https://www.webstaurantstore.com/advantage-chemicals-1-gallon-hand-soap-case/146HANDSOAP.html

https://www.webstaurantstore.com/advantage-chemicals-32-oz-liquid-dish-soap-case/146DISHSOAP.html

3) what's the (relative) safety factor of playing with dessert with regard to yeast infections? Like, is playing with anything sugary going to 100% give my girlfriend a bad infection?

1: if you use enough hot water, should be fine. Basically you want to dilute it as much as possible, then it will all just flow away.

2: we've done a liquid soap scene, no ill effects. Thinking of doing a few more, it was (good clean) fun! https://saturationhall.umd.net/download_info/veronicas-rainwear-and-liquid-soap

3: see previous posts on the subject by Candy Custard, has been posted about many times. Short version, wash well with clean water, but don't attempt to flush out and whatever you dp don't apply any soaps or cleaning products down there.

Honey and syrup are sugar based, and will readily dissolves into warm water.
Pudding and batter will eventually loosen and thin enough to flow down the plughole- however I find it easier to flush it down the toilet. (mix 50-50 ratio with water and swish together to make suspended lumps of goo.) The toilet can take it much better than the plughole.

My first few times were with soap/shower gel. No issues over the body- be careful over the head as it stings like fuck in the eyes and tastes pretty crap if you get some in your mouth.
Some soaps can also dry out your hair- bring plenty of conditioner.
--on a personal note- shower gel was fun, but doesn't hold a candle to proper gunge. It feels so much better and its much more eye/mouth safe to boot. Save the soap for washing off after

I also see dish soap in your list- I'd advise against this as its generally much more aggressive than hand soap/body wash. Skin irritation/dryness could become a problem.

Worth noting that some soaps can cause chemical imbalances in intimate areas.

Feel free to dig through my post history too- advice threads crop up fairly regularly.

I would read the labels carefully and maybe try a bit on your body before enveloping yourself

Have your eyes clear as well so you can see how her body is handling everything as well