The Steam version of FlatOut is the v1.0 version. This is the v1.1 patch for FlatOut that works with the Steam release. GOG is already patched to v1.1, and does not need this patch.
Originally posted by Clya900, the patch was missing 2 files are now included in this download, patch1.bfs, and patch.ini. Without these files, the game would fail to load, and shows a Runtime error. The game now has been tested on the Steam version, and works perfectly fine.