If you want to make a general reference to the Genstat statistical system, we suggest that you use the following:
VSN International (2019). Genstat for Windows 20th Edition. VSN International, Hemel Hempstead, UK. Web page: Genstat.co.uk
Alternatively, you could refer to a relevant Genstat Guide. These are all accessible from the menu bar by selecting Help | Genstat Guides.