Block&Parry System
This doc explained the block/parry related system.
Block Abilities
The GA_GCS_Block ability allows the character to enter GGF.State.Combat.Block state.
Using a montage for the block animation is optional, but it is recommended to have a block montage.
This also allows you to apply additional GameplayEffect (GE) at the moment of activation. This can be used to add extra information to the character, such as granting a TryingToParry tag that indicates the character is attempting to parry incoming attacks, thus triggering the parry action.
If you choose to use a block montage, you need to provide Start and End sections for your block montage:
When the block starts and ends, the ability will set the character's movement set to Guard through GCS_CombatEnityInterface. In your implementation, you should also change the character's animation blueprint to reflect the block stance when entering block state.