Specifically,
this is the entry GWG is referring to.
The Gold Dragon does not get DR 20/Magic until it is a Wyrm, which is older than Ancient, so he was (mostly) pulling your leg. However, DR means that he can resist damage (20 damage, in this case, nigh unbeatable by most TVtropes Super Weight 3 characters, even some SW4 characters), EXCEPT the kind of damage mentioned (In this case magic damage, out of a list of Epic, Good, Evil, Magic, Adamant; iirc). Which would be broken as E***. If it were DR xx/Epic, you'd need to be TVT Power Level 5 to beat it, even if it were only DR 5/Epic. DR 5/--, or no weakness, is unbeatable; however, DR 5 in chunky salsa equates to changing "You die" to "you are lethally injured, but alive for now". DR 10 would change it to "you are critically wounded, probably crippled". DR 15 is "You are badly wounded in the arm". DR 20 is "You are scratched".
However, he doesn't have DR 20/magic, so don't worry about that. If he's a Mature Adult, he's got DR 10/Magic, though. (Look under the Special Abilities column).
Gold Dragons have access to the spells in the Sorcerer list, as well as Cleric spells in the Law, Luck, and Good domains. However, it needs to be able to cast them, meaning having a caster level (look in the adjacent column) of at least twice the spell level (
Actually, look in Spells per day, if it's not "-" at that Sorcerer level, he can cast it). Also, that entry only lists Gold Dragon specific stuff. You can scroll up to the top for abilities shared by ALL dragons. Such as their breath weapon, spell resistance, and extrasensory abilities.
But there's a lot of
spells that he would qualify for, even as a Mature Adult.
To fit within TV Tropes Super Weight 3, he would be a Young Adult, though, at worst. DR 5/magic (lethal is usually still lethal, especially with magic), weak spell resistance, up to 2nd level spells, such as
Continual flame,
Acid Arrow, and
levitate, around the same skill level as, say, "(Lego) Harry Potter, year 3".