GTA V on the NX would be a system seller for me, I've been wanting to play a real GTA game on a handheld since VCS in 2006.
It would also sell very well. Nintendo only owners will get a taste of this great franchise, and I'm sure people who played GTA LCS and VCS on PSP would want another GTA game on a handheld. LCS sold 7.5 million and VCS over 4.5 million on PSP if I remember currently, and GTA V remasters do really well so I think it has potential to sell millions on NX.
If NX has Tegra X1 they could make a portable version that's much better than last gen versions, run it at 720p with lower anti aliasing than console versions due to smaller screen and enhance visual effects and framerate. Tegra X2 would be even better.