Depends on your budget. If you're looking to get a decent GPU without breaking the bank, I'd try for a GTX 1060. They may be a tad bit more expensive than the GTX 1050 or 1050 ti but it's worth. If you have a higher budget than $300 (£231), then I'd invest in something like an AMD rx 570 or above. All of these cards should be able to run GTA V at medium to high settings with 40-60 fps.