This depends on how you want to use gvm-tools. If you want to use gvm-cli you have to issue xml based GMP commands. The Greenbone Management Protocol documentation can be found at
https://docs.greenbone.net/API/GMP/gmp-7.0.html
If you want to write gmp python scripts you can use the python API provided by python-gvm
https://python-gvm.readthedocs.io/en/latest/
Some example scripts can be found gvm-tools/scripts at main · greenbone/gvm-tools · GitHub
Also take a look at the GMP category description for some more hints