Reddit reviews Edible Gold Leaf Sheets 10 Leaves 24 Karat S-size 1.0" X 1.0" Genuine Authentic for Foods, Cakes & Chocolates, Decoration, Health & Beauty, Home Arts & Crafts, Metal Working

You can try two things.

1) Kintsugi

This is a fancy repair that uses gold in the seams of the broken pieces.

This was the best site I found with practical instructions, even though in the end she wasn't successful. I feel a big takeaway from this is, if it's a piece you really love, practice on some junk first. https://alidoesit.wordpress.com/2014/02/05/kintsugi-hacked/

If you want to keep the cup as a decorative object, you can use an epoxy like J.B. Weld. This will be easiest to work with, and allow you to fill in missing bit.

If you want to try to use the cup (assuming there's no leaks), you need to use 100% food-safe silicone. It's not going to work as well to try to fill in the gap. You could trace the broken edge with gold and leave a gap; or you could put another broken shard of ceramic there (from your practice pieces, right?).

Silicone is food safe once dried. Other epoxies (ie: J.B. Weld) or glues (ie: superglue) tend to fall into a nebulous "non toxic" category, fine for handles or saucers, etc, anything not in contact with food/drink. Unlike glue or epoxy, silicone won't stick to itself once cured, so you'll have to do any broken sections that intersect all in one go.

For the gold, you can use food safe gold leaf or powder; or food safe mica powder. Either way make sure it's labeled edible.
Here's what I picked up, even though I haven't used it yet:

Silicone: https://www.amazon.com/00688-Household-Waterproof-Adhesive-2-8-Ounce/dp/B000KE4PBQ/

Edible Gold Leaf: https://www.amazon.com/Genuine-Authentic-Chocolates-Decoration-Working/dp/B00JGSWW5S/

2) Repair and paint

If you want to keep it as a decorative piece, you could also repair it with J.B. Weld (including the missing bit), and paint the seams to match the rest of the glaze.

Cwyn's blog has a couple posts about this:

http://deathbytea.blogspot.fi/2014/11/chip-and-how-i-fixed-him-but-good.html

http://deathbytea.blogspot.fi/2016/02/ruyao-ru-kiln-repairs.html?m=1

Either way you'll want to use a bowl of rice to hold your cup as you glue it back together. It gives you a "third hand" to keep the parts steady while you work. You should put together the parts one at a time, let the silicone or epoxy cure, then do the next part. Here's a good instruction on that:

https://www.teachat.com/viewtopic.php?f=87&t=12667&start=30#p173229

Good luck...