The most important thing to know is that the gold in Klimt's paintings is gold leaf, rather than the iridescent paints are available today. The larger online art supply stores stock gold leaf (for instance Blick), while the Society of Gildershas a list of more specialist suppliers.
In Pip Seymour's The Artist's Handbook he's got a couple of pages on using gold leaf with tempera, but you can adapt the information from the techniques described by gilders and, especially, icon painters.American Artist magazine has a useful feature on Fred Wessel, who uses egg tempera and gold leaf "to achieve Renaissance luminosity". Between all these resources, you should have enough information to start using gold in your own paintings.