Paper/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
@@ -54,9 +_,14 @@
         int i = 0;
 
         for (ServerPlayer serverPlayer : players) {
-            if (serverPlayer.setGameMode(gameType)) {
+            // Paper start - Expand PlayerGameModeChangeEvent
+            org.bukkit.event.player.PlayerGameModeChangeEvent event = serverPlayer.setGameMode(gameType, org.bukkit.event.player.PlayerGameModeChangeEvent.Cause.COMMAND, net.kyori.adventure.text.Component.empty());
+            if (event != null && !event.isCancelled()) {
                 logGamemodeChange(source.getSource(), serverPlayer, gameType);
                 i++;
+            } else if (event != null && event.cancelMessage() != null) {
+                source.getSource().sendSuccess(() -> io.papermc.paper.adventure.PaperAdventure.asVanilla(event.cancelMessage()), true);
+                // Paper end - Expand PlayerGameModeChangeEvent
             }
         }