"Does opening the iMac void the warranty"に関する回答It does not. If Apple can prove that when you opened up the case something got broke and you were the cause of it then yes . There is a reason computers have ram slots. To be able to add ram.
"Can you upgrade the GPU?"に関する回答Leo Leporte bought a 2013 MacPro and showed it on his TV show called Twit. (See YouTube) He stated it can be upgraded in his show. Along with the CPU Which other's have done already.