GVM is a full vulnerability scanning suite and is absolutely doing more then “just scan ports” (not sure where this info is coming from?).
You can find some high level notes about what GVM is doing here: https://docs.greenbone.net/GSM-Manual/gos-6/en/read-before-use.html
A list “what a default scan does” doesn’t exists and will never exist as such a list is getting outdated on a daily base (once new VTs are getting added to the feed) and is not maintainable.