The PDF is in the post right above yours. It just shows the wiring diagram for an open/close sensor which has The right type of dry contacts to be wired to the relay.
When the power on the relay fails (because the GFCI tripped) the battery powered contact sensor will then send the hub a message that it has been opened. So you just use the regular SmartThings app to create a rule for when that contact sensor opens.
SOME CANDIDATE DEVICES FOR THIS USE CASE
This is the Specific relay device being used in the pdf. It has both normally open (NO) and normally closed (NC) dry contacts. It costs under $10 and is a dumb device.
The Z wave battery powered contact sensor shown in that PDF is an older model and may be hard to find now. But you could use any contact sensor with dry contacts. It’s all the same idea. When the power fails on the device which is plugged in, The dry contacts inside the sensor Will cause it to change state. And because it is battery powered, it can still send a message to the hub that the state has changed.
Here’s the FAQ on newer devices with dry contacts. Remember that for your particular use case you need one that is battery powered, not an in-line relay.
FAQ: List of devices with dry contact input from external sources
The easiest to find will probably be the Aeotec, but it does cost somewhat more than most of the others. It should work well for this use case, though
BASIC METHOD
So just like the 2016 project report that I linked to in my first post, these all use the same approach.
A) Get a dumb device that plugs in to the second socket in the outlet that you want to monitor. This is typically a transformer or a relay.
B) Then get a smart battery powered device with dry contacts that you will use to monitor whether the dumb device in A) has power. when the dumb device loses power, The smart device will change state and report to the hub.
C) Then you can just write a very simple routine in the SmartThings app for when the Smart device changes state.
It’s your choice as far as what devices you use for A) and B). It’s a pretty straightforward method. 
Note that none of these set ups allow you to remotely turn the outlet back on after the GFCI trips: you still have to manually do that. But this will allow you to get a notification if the outlet loses power.