For a bunch of technical reasons, capturing the moment when a device goes off-line just isn’t really a good match to the home automation architecture that SmartThings uses. Particularly when it’s from a power cut. (No power means no way to send a message from the device.)
However, One clever workaround which people have used for years is to plug a device into the other socket of the GFCI outlet And set that device up so that it switches to battery power when the power is cut: and then it’s quite easy to get a report from that device to say that it is now on battery power. (If you use an open/close sensor, then typically you will get a report that the sensor has opened, for example.)
Here’s a project report from back in 2016. That explains how one person did it.
Simple way to detect power out
The smartest house used to sell a kit with the parts you need to do this, but it looks like they’ve discontinued it. Most people doing this kind of thing just put it together themselves.
A different approach designed for the new SmartThings architecture introduced in 2023 is in the following project report. This takes advantage of some of the newest architecture designs, but it will require having an always on server device on your system. So that’s a bit more work to set up and will also probably cost more than the “switch to battery” idea. So I would probably tend to recommend the second method only for someone who’s trying to track a device which isn’t as simple as an in wall outlet.