Then which will be the best GPU for it ?
Well, for GTA V? None, really, without significant upgrades elsewhere. You could buy much more powerful, but old GPUs like a GTX 660 or an HD 7870 (listed as GTA V's minimum requirements), but you'd still have an inadequate CPU on an ancient platform and are marginal with the RAM. And if your PSU is as entry-level as the rest of the parts, you'd need an upgrade there too to run a worthwhile GPU.
The basic problem is that while GTA V is an eight-year-old game, it's an eight-year-old AAA game, and it just won't run well on an entry-level office PC from about 2009. It's surely not the answer you want to hear, but you'd basically be looking at an entirely new build.
There are many wonderful games to play from the 80s up to about 2005 or 2006 that will run just fine on your PC. But AAA games from the 2010s? You just don't have the hardware for it.