The GPU in Mac Pro is professional and very powerful workstation.
For graphics, the 2013 Mac Pro has dual graphics chips from AMD's FirePro range. It hasn't said which cards are included, but from the quoted specs they appear to be the same chips as found in AMD's top-of-the-line FirePro W9000 graphics card – which feature 6GB of graphics RAM (and ECC RAM at that), a 384-bit memory interface and 264GBps memory bandwidth. Apple says that the cards will allow you to do VFX and editing work on full-res 4K video – and output the three 4K displays at once. Unlike the PC-based FirePro W9000 though, there are no DisplayPorts on the 2013 Mac Pro – instead the three of the six Thunderbolt 2 ports can be used as mini-DisplayPort outputs that output to DisplayPort monitors using an adapter, as with Apple's MacBook Pro and iMac.