Can I make a fork of the qbittorrent client with a gtk+ 3 frontend and call it deluge in the app store if I provide the attribution and links to the official developers?
As the other person mentioned, you would need to adapt the entire application away from Qt to use GTK. Vala is not strictly necessary, although it is commonly used.
Perhaps you could use Transmission as a base, or contribute further to Torrential?
As for naming the application Deluge -- I do not think this is appropriate, but you should credit the main developers anyway.