If you dig into the UE4 source code you will find a GameplayAbilities and GameplayTags module. These modules are not finished and there is no real documentation.
But some folks (like me) trying to use the modules to have a futureproof awesome working gameplay system. I have not the experience to build myself a system with this robustness and quality level, and i don’t want to create another system that is already implemented.
I thought this thread is the perfect place to collect as many information’s as we can about the gameplay modules. We can create something like our own community docs. This will help everyone to understand the gameplay systems.
Feel free to post links to information’s, or if you already understand the system, it will be epic to share your knowledge and explain a part of the system. 
Links to information’s that i found so far:
Official GameplayAbilities docs:
https://docs.unrealengine.com/latest/INT/API/Runtime/GameplayAbilities/index.html
Official GameplayTags docs:
About the general gameplay modules:
About GameplayAbilities:
About GameplayTags:
A little offtopic:
Iniside created an impressive gameplay system. (I dont know how good this is compared to the ue4 modules.)