This question was also already answered in GSF with OpenVAS? Or non-virtual enterprise edition? - Feed Services (GCF & GSF) - Greenbone Community Forum and GSF cost and arrangement - Feed Services (GCF & GSF) - Greenbone Community Forum where the following answer was given:
There is no way to buy a GSF for a homemade GVM setup.
It can be purchased only in combination with a Greenbone-approved/maintained environment,
namely Greenbone OS.