Gold Bricks are obtained in the following ways:
Complete a story level. Level Pack levels count towards this.
Rescue the citizen in peril (E.G. Hans Moleman in Meltdown at Sector 7-G)
Collect all the mini-kits in a particular level. This opens up an event in the adventure world for another gold brick. Note, The only events available are in the worlds for LEGO Movie (Through the level "The End is Tri"), Simpsons (2 events), Dr. Who (Level pack event only), BTTF (Main story level only), and Portal(Level pack event only). Ghostbusters and Midway Arcade (Main story levels only) can also have their events unlocked, though both are unavailable at the moment.
Achieve "Rule Breaker" Status. Applies to both the story levels and the adventure worlds.
Hidden around the adventure worlds through puzzles, quests, and races (May have to renovate to see the brick).
Simply collect any 30 golden bricks to unlock this achievement.