Congrats on the trailer. If you have what I would consider the standard mirrors on the GMC 2500, I think GMC calls them camper mirrors, they will be fine. If it is the standard mirrors the add-on aftermarket mirrors are good. I used the Grand Aero mirrors from made by JR products.
On my 2015 I have the Camper style ones that manually slide in and out and find that I don't slide them out. I can see down the sides of the trailer just fine. Shoot and post a picture and we can tell you for certain.
2015 GMC Sierra 2500HD Duramax