Compliance test mode in Gen1/2 and Gen3
The Detect state machine (in detect.detect) looks for a valid link partner electrically by (effectively) measuring the load impedance on a transmitter port.

If this is successful, the state machine will enter the polling state machine where it will start transmitting (and hopefully receiving) TS1 and TS2 ordered sets. This is true for all versions of PCI express.

The details of Gen 3 (different speed capabilities, different line coding) are done later in the sequence.