In Common GAS, you can choose to either use StateTree or the standard AIModule when it comes to build an AI system. I already provided you with some helpful Decoratorsand Tasks if you using the standard AIModule but if you're using StateTree, we dont provided any decorators and tasks as i havent battle tested StateTree enough but in the future, maybe i will provided some so you can still build your ai logic with StatetTree

In this example, im going to show you the basic setup to get you going.

Setup

By Default, CommonGAS already enabled AIModule ,StateTree and GameplayStateTree if you're planning to use that plugin and added them as dependecies.
For any AI-controlled characters, they need to inherit from CommonModularPawn and CommonAIPawnWithStateTree if you're planning to use StateTree

Next, you need to make an AIController inherit from CommonModularAIController to control the AI.

In the blueprint graph, on BeginPlay we simply call the Run Behaviour Tree and apply your behaviour tree asset.
Lastly, in your pawn class class default, find Pawn->Ai Controller Class and set your AIController to the one we created.