EDIT: Checked the WinSetTrans() and it only accepts a range of 0 - 255. Changed the two if statements to reflect this properly.
Func _Trans()
If IsHWnd($GUI) Then
Local $Key = ""
MsgBox(0 , "", $Trans)
ConsoleWrite(@HotKeyPressed)
Switch @HotKeyPressed
Case "{PGUP}"
If ($Trans + 5) <= 255 Then
$Trans += 5
ToolTip($Trans)
WinSetTrans($GUI, "", $Trans)
EndIf
Case "{PGDN}"
If ($Trans - 5) >= 0 Then
$Trans -= 5
ToolTip($Trans)
WinSetTrans($GUI, "", $Trans)
EndIf
EndSwitch
EndIf
EndFunc