You run a normal rift to get a trial key.

Then when you do the trial it'll give you a GR key based on how many waves you clear.

If you clear the GR within the time can upgrade your GR keys and do a higher level. It is not reusable.

IMO GR is more of an end game content for those that can already blaze through t6. I believe the drop rate for the trial key on t6 is 100%