How would i go about adding a animation ID to a gun script?

This requires you to use the UserInputService.

Also, to use an animation, you'll have to create it yourself, as ROBLOX blocks access to other people's animations for whatever reason.

Here's a small bit of code to help you get started.

local animation = Instance.new("Animation")
animation.AnimationId = "http://www.roblox.com/Asset?ID=Your ID here"

local animTrack = nil

function onKeyPress(inputObject, gameProcessedEvent)
	if inputObject.KeyCode == Enum.KeyCode.R then
		animTrack = player.Humanoid:LoadAnimation(animation)
		print("R was pressed") --You'll want to put your code here
		animTrack:Play()
	end
end