Transmigrated as a Ghost by MegaC

A 28 years old guy died in his room when sleeping for a reason that there's a gas leak in his room and then he got transported and now he's in a reincarnation space 'luckily' because he is a 10th million person who died he is given two choices reincarnate with no memories or transported to another world with boost start. Anyways you know what he chooses but he negotiated in how he want his transmigration be and then he is given 10 minutes to ask some questions. After that he is transported as a ghost first he acts like a 12 years old kid that this is all like a game and then after some ups and down he possessed the body of a 19 year old dead girl and because there's a small soul fragments left in the body of the girl he is now stucked in it, the only way to get out is to exact a revenge. After some childish beliefs and some adventures he killed the boss in his target revenge with his 'miracolously' power up and because of it his soul go boom but then he was saved by the administrator of the world and leave a few gifts like he can now materialise his soul body. After that the soul fragment of the girl thanked him and then he unknowingly absorbed it (it still resides in his soul). Skip from 80-90 Chapters. He fused with the soul of the girl he possessed since early Chapters and now he is both girl and boy but not genderless