+1I believe the GRC is more unique. Of course it has direct Corolla lineage since it uses a Corolla chassis, but it is more of a newer Celica GT-Four than anything.
Let's look at the facts:
AE86 - came in coupe or hatch, RWD, lightweight/barebones car, with N/A short stroke and high revving engine with Yamaha design (4A-GE)
FR-S/86/GR86 - coupe only, RWD, lightweight, N/A high revving, square engine - This car's development from the beginning was, fun lightweight rwd sports car and incorporated into its design was a callback or say fan service to the AE86.
Celica GT-Four - hatch, awd, turbo engine with strong internals capable of holding high hp comparable to its size - Created to compete in WRC
GRC - hatch, AWD, turbo engine ( possibly also strong internals capable of holding hp comparable to its size) - Beginning with the GRY was built to compete in WRC
Spec wise, the GRC is more of a successor to the Celica GT-Four but from a model name, the GR Corolla is successor to the Corolla GT-S (AE86). IMO, it was just the timing from when Toyota announced the Toyota 86 as the successor to the Corolla GT-S based on some of the ingredients of the 86 in similarity to the GT-S; NA engine, 2+2, RWD.
The GRC has taken lineage of both the Corolla name, combined it with the Celica GT-Four specs and bundled it into one. A lot rumors flooded about the return of a Toyota Celica and here we are essentially witnessing it now.