What's the correct glue to attach Gold leaf?

https://www.motherearthnews.com/diy/choosing-the-right-glue-zmaz03jjzgoe

Not glue. There’s special sizing to use. i am assuming you are using metal leaf, not real gold. The brand I️ have used is Wunda Size, but I’m sure any craft store will have sizing for metal leaf. Follow the directions

The “glue” is actually called size. The one that I use is Japan size. It appears to be a type of light varnish. It can be purchased from an Art Supplier. It can be purchased as a liguid or flakes. I have used Shellac to make a size by dissolving the flakes in methylated spirits. If using Varnish thin the liquid (with turpentine, if it is an oil based Varnish). Apply and wait until tacky (just tacky), then apply the leaf. If you require more details, I am happy to oblige. Other size materials used in the past (many years ago) are egg white and another is gelatin. I hope this advice is helpful.

I've used the classic gilder's.... japan size for over 50 years.... https://www.amazon.com/Winsor-Newton-Japan-Gold-Varnish/dp/B00681M2WY

Here is the new slow drying acrylic stuff..... https://www.youtube.com/watch?v=nTUlPyAt8JY I have a set of these I did over 40 years ago.