How is a gold rune different from a silver rune?
A similar question has been asked at BGG:
The collected wisdom is:

You can buy Gandalf cards with either the gold or the silver, or a combination of the two. Any time you need to discard runes, you can discard either or both.
The silver runes are gained through the spaces on the board, and you always gain one rune per space. The gold runes are stacked face-down (or otherwise randomized) so that you do not know which value of rune you are getting until it is picked. Only a few spaces on the board have you picking a gold rune.
Runes are good for scoring, Gandalf cards and can also be spend to avoid effects of at least one event. And 1 rune can be used to avoid a specific event tile (together with some other stuff). Might be some I have missed.