Gold makes reddit a pittance in comparison to advertising, unfortunately. It doesn't even make enough to keep the servers running.
Reddit and Twitter operate in the same hemisphere of community-driven applications — so it is within the realm of possibility that Twitter would have to bring some sort of "Gold" subscription model to their platform (who knows maybe it will be more widely adopted at Twitter). But all-in-all, they're both fighting the same battles: censorship, ad revenue slumps, and user growth. [2] And those are not easy problems to fix.
[1] https://www.reddit.com/r/howredditworks/comments/1oyhjw/redd...
[2] http://www.dailydot.com/via/twitter-moments-reddit-upvoted-m...
That's irrelevant, because most people don't pay for Gold. The important question is this: How much does Reddit make on Gold per user compared to how much they make from ads per-user? I sincerely doubt reddit is pulling in $4/user/mo in ads.