Similar to the silver badge "Yearling"(Active member for a year, earning at least 200 reputation); this gold badge is earned if "Active for one year, earning at least 2000 reputation".
Badges rewarded for reputation are unhelpful. Reputation is already prominently displayed on SE. Badges should be reserved for things that are helpful to the site but not directly rewarded with reputation.
edit to clarify: I consider reputation to be the primary measure of contribution to the site. I consider badges to be a secondary measure. Yes, I think reputation is helpful and should continue to be the primary measure of contribution. What I think is unhelpful is for badges to merely echo reputation gain.
I don't know the history or intention of badges like mortarboard.