Unlocking a gold chest on a level requires that you get a certain score. Score is earned by racking up combos of knocking gerbils below the line and from unused explosives. Also, earning a wipeout almost always guarantees gold, as you are awarded a significant amount of points for them.
Go HERE for a forum with some pictures and descriptions of where to place the explosives to get gold. Not all of the levels are included, but some of the more challenging ones are. Credit: Pencelgames.com.
A recent patch in early October 2012 added 12 additional levels to the game. These new levels also count towards these gold medal achievements, meaning you do not need to earn a gold chest on ALL the levels, just 72 out of the 84 levels.