papermc/paper-server/patches/sources/net/minecraft/server/commands/GameModeCommand.java.patch

--- a/net/minecraft/server/commands/GameModeCommand.java
+++ b/net/minecraft/server/commands/GameModeCommand.java
@@ -69,9 +_,16 @@
     }
 
     private static boolean setGameMode(CommandSourceStack source, ServerPlayer player, GameType gameMode) {
-        if (player.setGameMode(gameMode)) {
+        // Paper start
+        org.bukkit.event.player.PlayerGameModeChangeEvent event = player.setGameMode(gameMode, org.bukkit.event.player.PlayerGameModeChangeEvent.Cause.COMMAND, net.kyori.adventure.text.Component.empty());
+        if (event != null && !event.isCancelled()) {
+            // Paper end
             logGamemodeChange(source, player, gameMode);
             return true;
+        } else if (event != null && event.cancelMessage() != null) {
+            source.sendSuccess(() -> io.papermc.paper.adventure.PaperAdventure.asVanilla(event.cancelMessage()), true);
+            return false;
+            // Paper end - Expand PlayerGameModeChangeEvent
         } else {
             return false;
         }