Can the 2020 GMC Sierra 3500HD make a U-turn?

No. Needs more than 40 ft.

57ft curb to curb

Street width, feet

Street 40 feet curb to curb.

2020 GMC Sierra 3500HD Base LB RWD. Top view, to scale.

2020 GMC Sierra 3500HD: turning circle by trim, 40 ft street
Trim	Circle, ft	Margin	One move
2WD Reg Cab 142"	46.9	-6.9 ft	No
Base LB RWD	57	-17 ft	No
Base LB DRW RWD	57	-17 ft	No
2WD Double Cab 162"	53.9	-13.9 ft	No
4WD Reg Cab 142"	46.9	-6.9 ft	No
Base Double Cab LB RWD	57	-17 ft	No
Base LB 4WD	57	-17 ft	No
Base Double Cab LB DRW RWD	57	-17 ft	No
Base LB DRW 4WD	57	-17 ft	No
Base Crew Cab RWD	52.7	-12.7 ft	No
2WD Crew Cab 172"	57	-17 ft	No
2WD Reg Cab 142" SLE	46.9	-6.9 ft	No
SLE RWD	57	-17 ft	No
4WD Double Cab 162"	53.9	-13.9 ft	No
Base Double Cab LB 4WD	57	-17 ft	No
Base Crew Cab LB DRW RWD	57	-17 ft	No
SLE LB DRW RWD	57	-17 ft	No
2WD Double Cab 162" SLE	53.9	-13.9 ft	No
Base Double Cab LB DRW 4WD	57	-17 ft	No
SLE Double Cab LB RWD	57	-17 ft	No
Base Crew Cab 4WD	52.7	-12.7 ft	No
4WD Crew Cab 172"	57	-17 ft	No
4WD Reg Cab 142" SLE	46.9	-6.9 ft	No
SLE LB 4WD	57	-17 ft	No
SLE Double Cab LB DRW RWD	57	-17 ft	No
SLE Crew Cab RWD	52.7	-12.7 ft	No
Base Crew Cab LB DRW 4WD	57	-17 ft	No
SLE Crew Cab LB RWD	57	-17 ft	No
SLE LB DRW 4WD	57	-17 ft	No
4WD Double Cab 162" SLE	53.9	-13.9 ft	No
SLE Double Cab LB 4WD	57	-17 ft	No
SLE Crew Cab LB DRW RWD	57	-17 ft	No
SLE Double Cab LB DRW 4WD	57	-17 ft	No
SLE Crew Cab 4WD	52.7	-12.7 ft	No
SLE Crew Cab LB 4WD	57	-17 ft	No
SLE Crew Cab LB DRW 4WD	57	-17 ft	No
SLT Crew Cab RWD	52.7	-12.7 ft	No
SLT Crew Cab LB RWD	57	-17 ft	No
SLT Crew Cab LB DRW RWD	57	-17 ft	No
SLT Crew Cab 4WD	52.7	-12.7 ft	No
SLT Crew Cab LB 4WD	57	-17 ft	No
SLT Crew Cab LB DRW 4WD	57	-17 ft	No
AT4 Crew Cab 4WD	52.7	-12.7 ft	No
AT4 Crew Cab LB 4WD	57	-17 ft	No
Denali Crew Cab 4WD	52.7	-12.7 ft	No
Denali Crew Cab LB 4WD	57	-17 ft	No
Denali Crew Cab LB DRW 4WD	57	-17 ft	No

What this cannot tell you

Curb-to-curb turning circle as listed.

Mirrors and overhang need more room.