I am closeted trans boy and I would like mens shower gel or a deodorant. I need to tell my mum a valid reason why a "girl" would want or need soemthing like that. What can i tell her?
You could tell her it's better for your skin? Not sure on the truth in that nor if she'll believe you but it's worth a try!
You could tell her that you like the smell better. "Girls" soaps are often mire floral, while "guys" are often muskier. Or that you lost a bet with your friends and have to use email for 6 months or something. That usually works for me.
I think I would use the smell argument as well. I used Men's shower gel and deodorant way before I even knew that I was trans, precisely for that reason. Keep in mind that those things don't determine your gender though, nowadays only half of my products are explicitly advertised "for men", sometimes I just feel like smelling like a fruit basket.
Men's deodorant tends to be either cheaper or a larger size than the woman's due to less product being used, so you could say it's to save money.