I believe I’ve figured it out thanks to the BeagleBone Green Gateway specific documentation on the balenaCloud “Add new device” instructions.
After you’ve inserted the sd card with the BBGG powered down, you need to hold down the ‘BOOT’ button as it powers up. I also found unpluggling/plugging the USB cable as an unreliable way of powering up the device. So, it’s better to hold down the 'POWER" button to power off, and then hold down the ‘BOOT’ button and then press the ‘POWER’ button. You need to keep it held down until you see activity on the SD card activity LED.
I’m surprised this isn’t documented anywhere on the other BBGG getting started sites.