Example:
a standard climb gradient of 200ft/nm
divide 200 by 6000---3.3% gradient
another cool way is to just look at the climb speeds on the DP, find 100 kts and just below will be the climb per nm and that number is a good estimate for the required gradient. Here is a good article to study up on
http://terps.com/ifrr/feb97.pdf
good luck with the interview.