Have duels against Legendary Duelists Lvl 10. They are the weakest opponents in Duel World and require you to spend less Gate Keys.
What does "Use Skill 1 time" mean?
It is very simple, you just need to activate skill one time. If you do not know what skill is and how to set a skill to your deck, see the link below to GameA skill database. I would recommend you just set Yami Yugi's Power of Dark skill or Seto Kaiba's Peak Performance skill since those skills can be obtained when they reach level 4 and activate automatically at the start of a duel.
You have to unlock the Joey Wheeler guy at the gate with the gate keys. I think you have to get a certain amount of keys or be at a certain level in the game to unlock Joey Wheeler at Level 30-40 I think. I think you should maybe look at the strategy guides for this game on their websites.
Number of standard duelists increase and higher level gate duelists may happen at a lower level than stated here depending on your GX level