I don't think that there is a gui application. I am using nginx not for transfering files, but to have a local repository on device.
Don't know about java and never tried but there is this nginx-admin maybe it works and then you have a kind of gui.

Otherwise you could try to use ftp there is a gui but don't know if it is working https://openrepos.net/content/dipesh/ftp-server. But this is something similar what you are looking for.