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.
While the GRK itself is lost when used, if player beats the Greater Rift in time, Urshi may give them the new Keystone of higher level instead of upgrading Legendary Gems. However, choosing to upgrade gems, or failing the Rift, will not give such an option, forcing the player to find a new GRK.
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.