As its name implies, the concept of gender change refers to a being possessing a gender becoming a member of another gender of the same species. In reality it is possible to undergo surgeries and treatments designed to switch one's gender. People that undergo such procedures are referred to as transgendered, and may be of the male-to-female or female-to-male variety.
In video games and other forms of fiction, gender change is usually depicted with a much more fantastic or sci-fi bent. While there are limitations in how far transgender operations and treatments can go in the physical alteration of one's gender, such limitations are not present in fiction and thus the process can be depicted in a much more complete, instantaneous manner. One of the most well known subjects of gender change in fiction is the anime character Ranma Saotome of Ranma 1/2, who transforms from a boy into a girl when splashed with cold water and back again when splashed with warm water. Ranma and similar characters have been portrayed in a number of video games over the years.
Gender change should not be confused with the concept of cross-dressing, which is the act of dressing as a member of the opposite gender without actually physically becoming that gender.
Examples of Gender Change
To expand upon the above example, Ranma Saotome, the protagonist of Ranma 1/2, falls into a spring of cursed water, causing him to uncontrollably turn into a girl when splashed with cold water. The effect can be reversed with warm water. A near instantaneous transition, Ranma's male and female forms feature a noticeable difference in size and physical strength.
Resident Evil: Dead Aim
The male main antagonist Morpheus, upon injecting himself with the TG Virus nearing the end of the game, will transform into a female B.O.W.
Saints Row Series
The Boss, the player character of Saints Row 2, is the same character as the protagonist of the original Saints Row. The player is allowed to customize the character's appearance at the start of the game. However, while the player can customize the character as either male or female in Saints Row 2, the character creation system in the original game limited the player to strictly male design choices. In a sense, any game of Saints Row 2 started with a female protagonist has chosen to give the character a gender change. Additionally, the character creation system remains open after the main game has started, and the player can freely shift between male and female designs by visiting the proper locations. This same style of character creation carried over into Saints Row: The Third and Saints Row IV.
In Crystalis, the player plays as a male protagonist who at one point must find a way to enter a village of amazons. Entry into the village is impossible until the player gains a spell that transforms the player character into a number of different forms, including a woman.
The original Baldur's Gate includes a cursed magic item referred to as the Girdle of Femininity/Masculinity. Disguised as a simple belt, the item is found very early on, and when adorned causes the player character's gender to switch to the opposite. Otherwise, the effect is harmless. It is included as the developer's way of teaching players the importance of properly identifying items before equipping them.
In the Blood series, there is a cultist named Gabriel who is killed at the beginning of the first game. When he is eventually brought back to life, he is resurrected as a female; a cruel joke that plays upon Gabriel's homosexuality. As a woman, he is known as Gabriella.
The long-time Mortal Kombat antagonist Shang Tsung has the ability to morph into other fighters and use their moves. As such, any time he takes the form of a female fighter such as Sonya Blade or Kitana, he is temporarily changing his gender.
Demitri Maximoff of the Darkstalkers series has a special attack called Midnight Bliss. Part of the attack causes the opponent to undergo a temporary transformation. Any male affected is transformed into a (usually) sexually enticing woman.