For this achievement you will need to play for at least 2 hours on the same save file as both a male and female character.
You can switch your character's gender for $500 at any 'Image As Designed' stores. The gender you pick initially doesnt matter but I highly suggest that you start with the gender you want to keep first (Male for example) and just before switching genders you save your character at a clothing store (See
Saintified in Saints Row IV: Re-Elected)