[Answer]-I've created a custom User model. Can the CustomUserManager I create also extend django.contrib.gis.db.GeoManager?

Why don’t you create CustomUserManager class inherited from GeoManager and add the required user-related functionality there:

class CustomUserManager(GeoManager):
    def create_user(self, ...):
        ...

class User(AbstactBaseUser):
     ...
     objects = CustomUserManager()