A player can change their gender from Unknown to Male or Female. Once it is set to Male or Female, the player can't change it themselves. You can change it for them from their profile in the admin. You don't need to contact Nettla.
Change a player's gender
Go to Players → Player List and open the player.
On the Profile tab, click the pencil next to Gender.
Choose the New gender: Male, Female or Unknown.
Click Update.
Good to know
You need the Player list permission to edit a player's gender.
The change is made on the player's Nettla account, so it also shows at other clubs they play at.
Gender is used for gender-restricted matches, activities and dynamic cohorts.