How do you get a gerbil to climb ladders in it's cage?

You can't. Gerbils prefer to jump. Out of 13 gerbils, none have climbed ladders when ladders are in place (or they have jumped to halfway up the ladder and then jumped to the top). However, I've had 5 climb wire mesh when bored. My conclusion is that gerbils can climb if they want to, but much prefer to jump.