did requested changes

src/main/kotlin/net/axay/kspigot/gui/GUI.kt

@@ -19,7 +19,7 @@ class GUIData<T : ForInventory>(
     val transitionTo: InventoryChangeEffect?,
     val transitionFrom: InventoryChangeEffect?,
     internal val generalOnClick: ((GUIClickEvent<T>) -> Unit)?,
-    internal val onClose: ((InventoryCloseEvent) -> Unit)?
+    internal val onClose: ((GUICloseEvent<T>) -> Unit)?
 )
 
 abstract class GUI<T : ForInventory>(
@@ -99,7 +99,7 @@ class GUIIndividual<T : ForInventory>(
         if (resetOnClose || data.onClose != null) {
             listen<InventoryCloseEvent> {
                 if (data.onClose != null && playerInstances[it.player]?.bukkitInventory == it.inventory) {
-                    data.onClose.invoke(it)
+                    data.onClose.invoke(GUICloseEvent(it, playerInstances[it.player]!!, it.player as Player))
                 }
 
                 if (resetOnClose) {

src/main/kotlin/net/axay/kspigot/gui/GUIBuilder.kt

@@ -47,7 +47,7 @@ class GUIBuilder<T : ForInventory>(
 
     private var onClickElement: ((GUIClickEvent<T>) -> Unit)? = null
 
-    private var onClose: ((InventoryCloseEvent) -> Unit)? = null
+    private var onClose: ((GUICloseEvent<T>) -> Unit)? = null
 
     /**
      * Opens the builder for a new page and adds
@@ -66,7 +66,11 @@ class GUIBuilder<T : ForInventory>(
         onClickElement = onClick
     }
 
-    fun onClose(onClose: (InventoryCloseEvent) -> Unit) {
+    /**
+     * A callback executed when the user closes
+     * the inventory.
+     */
+    fun onClose(onClose: (GUICloseEvent<T>) -> Unit) {
         this.onClose = onClose
     }
 

src/main/kotlin/net/axay/kspigot/gui/GUICloseEvent.kt

@@ -0,0 +1,10 @@
+package net.axay.kspigot.gui
+
+import org.bukkit.entity.Player
+import org.bukkit.event.inventory.InventoryCloseEvent
+
+class GUICloseEvent<T : ForInventory>(
+    val bukkitEvent: InventoryCloseEvent,
+    val guiInstance: GUIInstance<T>,
+    val player: Player,
+)