on the 2015 you need to get a io5 or io6 radio and hmi 2.0 or 2.5 (the 2.5 has built in nav) you would then need a gps antenna (cheap) if you intend to use nav
if you want to do it cheap you can do a little research and get them used off of ebay and use a obd genie to unlock them.
if you do that I highly recomend getting the module out of the same make model vehicle, chevy for a chevy, gmc for a gmc.
some modules can come out of other vehicles like a smaller level suv, so for ease of function I recomend getting it from the exact same model. tahoe for a tahoe, etc
the only other catch is if you have the small screen that has to be upgraded also so there could maybe be a harness involved
some places online offer a "kit" but you are going to pay $600-1000, you can diy with normal functionality for half that cost
a obd genie is like $150, a hmi can be found for $150 or less you just have to shop around, havent checked radio module prices but they are probably similar to hmi prices
so figure around $450
*note that if you diy and later decide to have it programmed with SPS for some reason you will need a VCI number to be sure the upgraded modules are programmed correctly otherwise it programs the modules back to what you had originally.