Try googling gta sa widescreen-fix, should be a .asi or cleo-file.
That is only for the gun target.
If your resolution isn't showing up, then there are two possible issues:
1. You are looking at 16-bit resolutions by mistake.
2. You need to update your video drivers, or install ones specific to your video card/on-board video, instead of using generic/plug-and-play ones.
From there, you'll need to set your game to the proper resolution, and then depending on how it displays, you may or may not wish to enable the "Widescreen" feature in the game's video settings.