Not knowing exactly what truck you have...here's what my owners manual states.. BTW, there is not any difference between a 2013 and 2014 GMC truck.
2WD (C2500)..extended cab S/B 5er 17500#.....GVWR 24500#
2WD " crew cab " " 17400# same
2WD " regular cab L/B 5er 17800# same
2WD " extended cab L/B 5er 17400# same
2WD " crew cab L/B 5er 16700# same
If you have 4WD the weight rating drop some....the lowest is 14700# for a crew cab L/B, but still has GWR of 24500#. Nothing I read states that 20" wheels make a difference. I'm not sure what has you worried about the tires. Maybe a quick trip to your dealer will answer you questions and ease your mind.
P.S. I worked for GM for 45 years....I know their trucks.