Hi, the process starts from chapter
102, where the scientist girl is realeased from possession, then the demon merges with a monster summoned and thus becomes a GTS monster.
Later on(in chapter
104) when Baki and the remaining demons give her a hard time, she gains more power and an FMG powerup happens.
Ch102:
https://lhscan.net/read-jigoku-sense...apter-102.html
Ch104:
https://lhscan.net/read-jigoku-sense...apter-104.html