Same way as its always been in PocketMine. Example PHP: $player->sendPopup("Hi. lmgtfy rules"); Where $player is obviously an instance of the player.
please give us a more in depht explanation of your GUI anything from screenshots to in detail text helps for now i can assume you meant trading GUI chest GUI anvil GUI hopper GUI or what?
Hmmm. You need to elaborate what Tile you need so they could understand you... now you've elaborated it perfectly.. Its a Chest Tile btw. The term GUI doesn't sit perfectly