Download the 12GB ISO. Install the latest xliveless wrapper. Lock your framerate to 60fps (via GPU control panel). And enjoy the best racing game of 2008 the way it was meant to be played.
The modding community (GRID Mods, RaceDepartment) built their tools around the . The later v1.3 patch broke custom tracks and car skins. If you download the untouched original FitGirl repack , you get the v1.0 executable untouched, allowing you to install the legendary "GRID Enhanced Physics Mod" and the "2009 Le Mans Series Total Conversion" without crashes.
Original ISOs for GRID could be bulky with duplicate files. FitGirl compresses these down significantly. You can download a smaller file and install only the components you need (e.g., skipping multiplayer files if you only want the career mode).