.GCF File Extension
What is a GCF file?
Cache file used by Valve's Steam game distribution software; contains a virtual file system that may include file fragments, metadata, checksums and other game information; used for storing downloaded games and game updates from the Valve distribution service.
GCF was initially short for "Grid Cache File," but now stands for "Game Cache File." It is the format used for storing official Steam-integrated games and for downloading official content. GCF files are not meant to be manually altered by the user.
NOTE: Valve released the Mac version of Steam in May 2010.