The Goal Structuring Notation (GSN) Standard:

This Standard has two intended functions:

• Firstly, it seeks to provide a comprehensive, authoritative definition of the Goal Structuring Notation (GSN).  
• Secondly, it aims to provide clear guidance on current best practice in the use of the notation for those concerned with the development and evaluation of engineering arguments  – argument owners, readers, authors and approvers.

Current Version of the GSN Standard:

The latest version of the Goal Structuring Notation (GSN) standard (version 3) is now available.

Earlier Versions of the GSN Standard:

An overview of the key changes from GSN standard V2 to V3 is available: (here)

Version 2 is available here 

Version 1 is available (here)