The GAGE timer is guaranteed available, in that you can download it now and it will always be available to you. Its also as portable as JOGL, in that it works on the standard gaming OSes.
High resolution timing is essential to many things, putting it in every API that its essential to would create a maintainence nightmare.
Why implement yet another timer for the sake of it?
GAGE is not guaranteed always available - the project could cease to be supported.
Additionally, if I distribute a Jogl application currently, I only need ensure that the user has Jogl installed - adding GAGE to the user instalation just to get a timer class is onerous.
Why implement a timer? Because it is a core feature of any 3d engine. It seems to me like the utils package is an obvious place for such a class.