How to invite other gym trainer in Wyndom Stadium?

Solution 1:

To be able to invite each gym leaders (also Leon, Hop, Bede, and Marnie), you need to get their gold cards.

• You need to have finish the post-game adventure, for next trainers :

  Leon : Battle him at Battle Tower
  Hop : Finish the post-game by battling him, deep inside Slumbering Weald (You must have begin the post-game part by beginning at this place too, the endgame show that we must return to this place)
  Marnie : You need to find her in Budew Drop Inn, then go rebattle her at Spikemuth after the post-game adventure, next to the Pokemon Center

• These one are given during the post-game adventure :

  Piers : You get it during the post-game
  Post-game Gym Leaders : you get their gold cards when you go to their stadium during the post-game

  Note that Bede is the new Fairy Gym Leader

• Unknow location :

  Opal : I don't remember when or where you get it, probably during post-game