Use the key on the nephlalem obelisk m8.
You'r given a choice on which key to use on the obelisk (I.E normal rifts keys, KoT or GR keys)
All keys can only be used on the obelisk (whether it be normal rift keys, GR keys AND also the KoT)
As i said be4, KoT (Key of Trials), are obtained via defeating regular rift guardians.
Put that key on the slot on the obelisk and you'll go through a trial run of various enemy waves to determine what level GR key you get.
Once you've completed the trial, you'll be awarded a GR key of a specific level (as in GR level 10 and so on) which is determined on how well you do on the trial.
Once again, use that GR key on the obelisk and the GR will open for ya.
Hope i was clear enough.
But where do I use the Key?