Gender guessing is a simple statistical function of Given name.
You must have more people with their given name defined as ‘Private’ where the gender of ‘female’ has been selected.
You don’t need to use aliases for the Given name.
If you set the ‘Private given name:’ in the Text tab of Preferences to ‘PRIVATE’ then reports will redact the Given Name if the Padlock icon is toggled to Private in the Name Editor on Reports. (Or if you use the ‘Living’ option.) This is different than the Private toggle (padlock icon) in the Person Editor. If the Entire Person is toggle to Private, then that Person is entirely ommitted from the Reports. (Privacy can be override for Reports & Exports.)
Note that Privacy only affect output (Reports & Exports) of Gramps. Privacy is shown as a padlock but otherwise ignored in the Views.