Make sure you have Garry's Mod (64 bit) ininstalled.
If not close game, right click garrys mod, properties, betas, choose x86-64. Let it update.
If that has been done you'll need to get Garrys Mod back open, go to your installed maps and uninstall San Andreas map from garrys mod.
Once it's uninstalled click install, you should get a prompt to install an addon dependency (a base map) that needs to get installed (press the + ) and then press confirm. Then the install of San Andreas should go. Then run the map.