ok, then, from gta press the pause menu, while gaming it
on the settings tab select graphics
there you will see the resolution, should be 1280x720, so click on the little arrows on the right of that number untill you see 1920x1080
the game will ask you permission to apply those settings, it might ask you to restart the game, accept it
when it restarts see the resolution, if it looks sharper than the lower resolutiuon, that was all
if you see the game now runs slower, you will have to play with graphics settigns and game effects, on the same menu you changed resolution
this part doesn't depend on them onitor, it depends on the gpu and your cpu