Please have a look here and follow the PDF linked there:
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.
There are commercial virtual appliances offered by Greenbone that do offer the GMP API.
Currently there is GSM ONE, and soon two further virtual products are going to be launched,
one positioned below GSM ONE, and the other positioned above GSM ONE. You can operate all of them headless.
See here for an overview: