The StandardSDKSource_Linux.tar.gz file you have requested: Tools and Utilities / SDK / Linux-Friendly SDK - 568.51kb is not available. Perhaps try browsing all files for the game Half-Life and upload any files you want to share or are missing. This is the only information we could locate:
Standard SDK source code that is Linux-friendly. If you are working on a mod based on the Standard SDK and you would like to release a Linux version, you might be interested in checking this out.