I did Gold coating by the electrolytic process for about twenty years as an aside to my polishing and engineering work. It only works on certain metal mirrors such as steel, copper, and nickel. It involves working with Potassium Cyanide and Gold Cyanide and is not for the amateur to tackle unless unless prepared to shell out thousands of dollars for the Gold solutions, filtration units, polypropylene holding tanks and stainless steel cathodes. I have done up to 30-inch aluminum mirrors with an electroless nickel layer applied before polishing.
For glass mirrors, Gold can be applied by the evaporative process, very similar to aluminizing and a Mag or Thorium Fluoride overcoat can be applied for scratch resistance. It is very soft like bare aluminum and sleeks quite easily. Only non-contact cleaning can be used.
Sometimes an underlayer of Chromium is applied first to improve adherence, otherwise the bare Gold may fail the scotch tape adherence test.
Reflectance is nil in the Violet but runs up to 0.98 at the red laser wavelength of 633nm. It gets to 0.99 in the thermal IR.
I don't believe there is a chemical deposition process for glass as there is for silver.