Historically, these files were distributed via community forums or "FastDL" (Fast Download) systems. To use the contents:
Custom character skins or prop textures. HG MOD.rar
As with any .rar file from a third-party source, these should be scanned for malware, as executable scripts within certain game engines can occasionally be exploited. these should be scanned for malware
Many versions found on old forum mirrors may be outdated and incompatible with modern Steam versions of games. HG MOD.rar