Fill procedure is in the manual, but as above if it's only down a little just add some and it should self correct after a couple of heat/cool cycles.
Common to have to add a little to a new one.
What year is the truck?
How many miles/how long in service?
2018 GMC 2500 L5P CCSB 4x4 Denali - Mineral Metallic; previous '11 LML, '10 LMM and 04.5 LLY