getting cannot convert to male error in django?

You can get the choice display (so here the value for female_label and male_label) with the .get_gender_display() method, so:

customer.get_gender_display()

Indeed, as the documentation on the .get_fieldname_display() [Django-doc] says:

For every field that has choices set, the object will have a get_FOO_display() method, where FOO is the name of the field. This method returns the “human-readable” value of the field.