that's difficult to say. At 720p you're not using much vram, so that won't be a problem. So you will be able to adjust some settings up! With that said both of those games are notorious for inconsistent performance.
For GTA v there are some scenes that bring even a 1070/1080 to it's knees and frames drop well below 60fps (60 being the magical smooth game-play mark). For PUBG it's kinda similar. Both poorly optimised games for current hardware capabilities.
I'd expect you to be on or near the mark though. So prob 60fps+ @720p with high settings, but expect some occasional dips in FPS on really demanding scenes. For those types of games, I'd suggest disabling things like AA/AF/Post processing. They are all demanding settings for a GPU, and then set the remainder settings at high, and you should get consistent FPS at that res. Nice and smooth