There is a risk towing in 6th gear unless you can maintain at least 1700-1800 rpms.
Below that you’ll be lugging the engine, monitoring your ECT, Trans temps and EGT’s will show 6th towing isn’t a good idea.
2007.5 GMC "Black" Sierra SLT 3500HD SRW C.C. 4X4 LMM Z71
DIY stock airbox mod, aFe ProGuard 7, OEM drop-in
EFILive ECM ATP - Heavy Tow Tune with Turbo Brake
EFILive TCM Duramax Tuner
DHD Cold Side High Flow Intake Pipe, DHD PCV Re-route, AirDog 4G100
Castrol Hypuron/Vecton 15W40 synthetic blend oil
Donaldson P550832 2 qt oil filter
2007 GMC Yukon Denali "BLACK" 6.2 gasser, DIY stock airbox mod, K & N OEM drop-in, Airaid intake tube, EFILive TCM tuned