The Gameplay Abilities System in unreal is an incredibly helpful plugin that makes the creation of gameplay behavior easy.
By using the Gameplay Ability System on can create complex networked behavior in a simplified manner.
Unfortunately there are not that many official resources to learn the UE5 Gameplay Ability System, so here are some useful links that may help you out.
GAS Documentation Github - The most similar thing to a documentation of GAS, if you are not familiar with Unreal GAS start here with this Gameplay Ability System Github repo
Using the Gameplay Ability System - In this talk Matt Edmonds of Splash Damage Games helps you decide if GAS is right for your team or project.
The truth of the Gameplay Ability System - Alvaro Jover talks about the Gameplay Abilities System in Unreal 5, I recommend following him as he shares good information regarding Unreal Engine
A Guided Tour of Gameplay Abilities - An official video by Epic on the usage of GAS in Unreal Engine
GameplayPrediction.h - You will need access to Epic github or have Unreal Engine source code downloaded. This .h file explains really well how Network Prediction works in the Unreal Engine Gameplay Ability System
These are some of the first resources I would redirect someone that wants to learn about GAS.
If you have any question don't hesitate to ping me at @QuodSoler with any question.