Getting a GRK requires player to obtain the Keystone of Trials from any Rift Guardian in a regular Nephalem Rift first, then complete the Realm of Trials. Doing so will grant a GRK with rank proportional to the number of waves cleared.
The GRK is not lost if player beats the Greater Rift in time: Urshi may upgrade it to higher level instead of upgrading Legendary Gems. However, choosing to upgrade gems, or failing the Rift, will destroy the GRK, forcing the player to find a new one.
To participate in the Greater Rift run, every player must contribute a GRK of equal or higher rank compared to the first one used.
Greater Rift Keys are not unique; the player may have as many as they can obtain at once, and must choose which of them to invoke and upgrade every time.