Add Keys#PREVIOUS_GAME_MODE

src/main/java/org/spongepowered/api/data/Keys.java

@@ -2181,6 +2181,11 @@ public final class Keys {
      */
     public static final Key<Value<Integer>> POWER = Keys.key(ResourceKey.sponge("power"), Integer.class);
 
+    /**
+     * The previous {@link GameMode} of a {@link Humanoid}.
+     */
+    public static final Key<Value<GameMode>> PREVIOUS_GAME_MODE = Keys.key(ResourceKey.sponge("previous_game_mode"), GameMode.class);
+
     /**
      * A {@link Beacon}'s primary effect.
      */

src/main/java/org/spongepowered/api/entity/living/player/server/ServerPlayer.java

@@ -344,6 +344,15 @@ public interface ServerPlayer extends Player, Subject {
         return this.requireValue(Keys.GAME_MODE).asMutable();
     }
 
+    /**
+     * {@link Keys#PREVIOUS_GAME_MODE}
+     *
+     * @return The previous game mode of the player
+     */
+    default Value.Mutable<GameMode> previousGameMode() {
+        return this.requireValue(Keys.PREVIOUS_GAME_MODE).asMutable();
+    }
+
     /**
      * {@link Keys#STATISTICS}
      *