Hello guys.
This guide contains all the fighting styles, with their prices and a small preview, there's also where and how to purchase a custom fighting style, you should head to Ganton gym (SS of the door below), type (/enter) at the door and you can then purchase a custom fighting style using this command (/train) at the "i" icon (SS below in the spoiler), then you'll have a full list of fighting styles, and choose the style you want, they cost $50,000 except the normal one, It's free.
Spoiler!
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Credits goes to Bilal™ .
Thanks to Alvin™ for helping me to capture those videos.