How to trigger a sampling oscilloscope on a PAM-5 signal
Hm... Observing (measuring) the 1000BASE-T physical waveforms is not so simple (and obvious) as with 10BASE-T and 100BASE-TX. The difficult is from the fact that the 1000BASE-T PHY uses bidirectional (and one-source synchronous) transmission at each of the fours pairs being employed.

Therefore, to test a signal of the PHY, you need:


configure the PHY to a special, test mode when it will generate a known waveform on a pair, needing no link partner: there are five IEEE 802.3ab test modes described in IEEE Std 802.3, Section 40.6.1.1.2, one/some of them may be of your interest;
if your PHY IC support the test trigger(s) outputs on its pin(s), e.g. like National Semiconductor DP83865 does, you may use such a trigger output to synchronize your oscilloscope with the test waveform you selected in the PHY; or
if your PHY IC does not support the test triggers, your need use a native means of the oscilloscope, but imo it is not problem now (with known, one-directional waveform) because the test waveforms are single-sourced and generated repeatedly and/or continuously.


Also one thing to remain is that to do all these things, you need a configuration access to the PHY under test, e.g. MDIO/MDC or something else to control the PHY registers to reach the necessary behavior on the wire.