When the lender does not care which Treasury it receives, the trade is GC and the rate is the baseline funding rate for the market. GC is the benchmark against which specialness is measured.
GC repo rates should trade near iorb and the top of the target-range. Persistent GC pressure above that band signals scarce reserves and is one of the things that triggers use of the standing-repo-facility.
Example: GC repo prints at 5.33% while iorb is 5.40%. A specific 10-year note trades at 4.85% in repo, so it is 48 bp special, meaning holders of that bond can fund it far more cheaply.
Related: repo, special-repo, sofr, iorb, standing-repo-facility