You'd have to narrow down the model of GM ECU and mated ignition coil modules.
ECU ignition driver outputs are generally push-pull 5V current-limited to 20mA, with protection so a short to +12V will not backfeed and raise the ECU 5V rail, destroying it.
The ign module typically has a smart coil-driver IC and IGBT inside.
Some models of ECU and coil, the ignition coil module can (over the 5V trigger wire) send back diagnostic info to the ECU, such as current-limit target reached, over-dwell timeout.
The ECU can also look for its (trigger) output shorted to GND or shorted to +12V battery, or open load.
So the trigger line is not only a digital output but also can have switched current-sinks (by the coil module) on it, for diagnostics.
If you want to fully test ignition coil modules, it's not just the spark but also the diagnostic signals to consider.