Gacha Gacha - Secret
https://www.mangaupdates.com/series.html?id=27751
Description:
Hatsushiba Akira, an awkward high school senior boy, has a strong crush on classmate Sakuraba Yurika. But when he gets close to her, he always says or does something stupid.
At the suggestion of his best friend, Kikuchi, Akira enters the GachaGacha machine to play a game, but the machine fritzes out and malfunctions. That night, he sneezes while going to bed, and when he wakes up, he looks like the female avatar he saw in the GachaX2. He soon learns that sneezing will change him back and forth from his normal self to a female form. As a girl, Akira-chan is able to get close to Sakuraba and to become her friend.
(55 chapters long)