You don't want an insert. A spray in bedliner may be what they were referring to however what you want for sure is a rubber bed mat. They are available through automotive retailers or some folks use mats from farm equipment stores.
You can search this site for more info.
'11 GMC 3500hd SLT DRW D/A 4x4 Spray-in, Air Bags w/comp & controls, TL, Superhitch, 48" Supertruss, 9.5t Warn, front rcvr, Fox Shocks, '11 AF 1140, DB, AC, Gen, Solar, Thermal Windows, Wardrobe, Rear & Side Awnings, 18' Lund/40hp Honda 4 stroke