Why I cannot wear fashions in my gunner?

That's a solution only for your client. Other people can have 'Detailed' on so they will only see you as one of the many 'clones'.

Customization for new classes was patched in PWCN already, but I don't know the details(can't read Chinese and people that can around here and that interact with the PWCN client for some reason don't bother covering fashion/customization in any semblence of depth =/)