• The minimum difficulty needs to be Very Hard.
  
• The gnome MUST be secured by being thrown, not carried at the chopper, not in the kitchen elevator.
  
• It also needs to be done in loud, acquiring the gnome forces loud anyway

After playing the heist for a few times, it appears that the gnome will spawn in one of the two cars in the garage (see below). Look for a car with a glowing red alarm light behind the windshield, then just open the trunk and bag the gnome.

Opening the trunk triggers the car alarm (obviously) and forces the heist into loud. Despite his small size, you can't run while carrying him on the back.

Once you take him, just carry on with the heist as usual. Make sure you have him on your back when using the ziplines in the elevator shaft. Once the helicopter arrives, throw him in. Don't carry him while ending the mission, it needs to be secured by being thrown into secure area. Don't throw him into the kitchen elevator, the achievement specificaly requires that you secure him in the chopper.

You get the achievement upon entering the escape zone.