I've never heard a "Gold" ATIS. Us Heathrow chaps are, of course, used to the word "Gold" when ordering bathroom fittings, car ashtrays, etc.....
The ATIs provides, among other things, information about the weather. Weather reports at airfields are updated usually once every 30 mins - more frequently if the weather is bad. Each ATIS broadcast is given a code letter which, as you may have tumbled, follows alphabetical sequence. Hence the first broadcast of the day is "A", next is "B", etc. I would have thought that this would have been obvious if you listened regularly..
The ATIS broadcasts change as often as is required to update the information but once every 30 mins is about the norm.