The user can alter ones and/or other's gender either permanently or temporarily, they may be able to combine genders for hermaphrodite or remove both for neutri. The change is complete to the cellular level.
Salmacis' pool (Greek Mythology); into Hermaphrodites.
Trans State (Sinfest)
Kukri's Pocket Dimension (SNK Heroines: Tag Team Frenzy)
The Incubus (The Black Room) used a demon ritual to transform himself...
... into a Succubus so everyone would desire her.
Willow Rosenberg (Buffy the Vampire Slayer) in the process of casting a gender change spell.
Prue Halliwell (Charmed) was transformed into a man after her sisters casted a spell meant to attract a Succubus.
Amorpho (Danny Phantom)
Erik Storn/Amazing Woman (DC Comics)
With complete mastery over his demonic body, Muzan Kibutsuji (Demon Slayer: Kimetsu no Yaiba)...
...can easily change his gender.
The Hive (Destiny) can change their sex at any point in their lives for reproductive purposes.
The Doctor (Doctor Who) regenerates into a female for the first time.
Alucard's female form (Hellsing).
Thanks to the frog DNA used in their creation, the original cloned dinosaurs from InGen (Jurassic Park) could change their gender in a single-sex environment.
Superman (Justice League Action) transforming into a woman due to the effects of Pink Kryptonite.
As she/he is gender fluid and a child of the shapeshifter Loki, Alex Fierro (Magnus Chase and the Gods of Asgard) changes gender spontaneous and at random, which she identifies as female most of the time, he occasionally has “very male days”.
Courier (Marvel Comics) has the ability to control and 'talk to' every cell in his body, allowing him to...
...transform into a woman, among other things.
Curtis (Misfits) as Melissa.
Emporio Ivankov (One Piece) alters his gender.
Orochimaru (Naruto) could become either male or female depending on certain circumstances. This has officially made him a non-binary character.
Seiya (Sailor Moon) changes from male to female.
SCP-113 - The Gender-Switcher (SCP Foundation) is a polished piece of red jasper that is capable of transforming any species with sex chromosomes into either male or female genders via direct contact.
The player (Terraria) drinks the Gender Change Potion.
Gnome (Valkyrie Crusade) can change her/his own gender whenever he/she wants.
Max (Wizards of Waverly Place) being turned into a little girl.