I use lots of different gold products depending on the look I want.
For spraying (to get every recess in a cookie or to stencil), I use luster sprays like PME or Pavoni (an Italian brand I recently discovered, which I love because it's super shiny). Of course, airbrush coloring is also possible here, but I am often too lazy to get out my airbrush.
For sponging (just getting isolated gold highlights on raised elements) or painting small areas, I like to mix luster or highlighter dust with a little alcohol-based extract and then I either use a sponge brush or small craft paint brush to apply it, depending on the technique. Highlighter dust is MUCH brighter/shinier than luster dust, but it's labeled for "display only" so I don't use it other than on "show" pieces.
Then, there are gold Rainbow Dust metallic pens for drawing finer lines . . . Oh yes, and edible gold leaf. I lift small pieces of it from the sheet and stick it onto cookies with a touch of corn syrup. Used it on my decoupaged eggs in a recent video. I'm sure I've just scratched the surface, but these are the ways I most often apply gold.