You don't have to do anything to a kit in order to enter, as long as it's within regulation. The main thing is you must use official Bandai Gunpla or Gundam-themed kits, including decals. Things like putty, pla plate, LEDs, and other "general modeling materials" are also permitted. Official regulations here: https://gbwc.bandai-hobby.net/en/regulation/

Here's the finalists from last year's tournament: https://gbwc.bandai-hobby.net/worldfinalist/result.php