This is old version of Thirteen AG's Widescreen Fix which actually works better than recent version when combined with Widescreen HOR+ Support mod, so make sure you download it as well to make it look better.
https://www.gtagarage.com/mods/show.php?id=25380
Supports 1366x768,1920x1080 as well as 2560x1440 and up.