Is the Play Montage node not working for you?
Edit: Oh wait. I think I see what you mean. You want to sync the animation with your character’s animation? Yeah, that’s a bit tougher than just playing the montages together. You’ll probably want to use some IK setup on the characters’ hands to hook his hand to the gun mesh at a specific socket, but honestly I’ve never done it, so that’s about all I can give you. There’s got to be some tutorial or something out there though.