I had difficulties following that guide too; eventually I concluded that it was deprecated, as you’ve found. I couldn’t find any decent information anywhere.
So I’ve written the following tutorial on the UE wiki:
If it doesn’t solve your problem directly (the guide is intended for use with a UActorComponent) then it should at least form a basis for you to base something of your own on. It is quite brief the minimum amount of code you need to implement a your own specialised FGameplayDebuggerCategory, but what with the serialisation approach it’s not easy to understand. Regardless, I expect the guide should show you the (new) way to do things and get you on your way.