In order to gain access to this level you must first gain 100% game completion by getting all 12,000 gems, rescuing all the dragons and capturing all the eggs. Once you do this, you unlock the level Gnasty's Loot
available in the Gnorc Gnexus
Inside the level, you must collect all 2,000 gems to unlock this achievement. This level features the ability to freely fly around, but you can only fly as high as the highest point you've reached in the level. Work through the level, using Sparx to point out things you may have missed. Upon collecting all 2,000, the achievement will unlock.
If you're looking for a walkthrough of this level, you can find one in the video below at 3:04:57
. The video also features all possible achievements in the game.