Gold in Virgil's poem symbolizes what is special, hidden, and rare. This can be seen in the golden bough stashed away in the deepest, darkest corner of the forest, which only the designated person (Aeneas) can take to guide him in the underworld. (Though, of course, there's that whole ambiguity over whether it comes off the bough of its own accord, or whether he has to snatch it off.) Through the motif of the "Golden Age" which faintly persists in the civilization of the Arcadians on the future site of Rome, it increases the sense of the Trojans' homeland as secluded and important. You can see how significant it is when Virgil tells us that Caesar Augustus is going to bring about a new golden age – he's going to make the rarest and most special thing available to everyone.