Is there a way to link an animation made in Animation Editor to a certain key?

USER INPUT SERVICE

UIS, aka User Input Service is the best way to go. UIS is used in local scripts and can be triggered by InputBegan, InputEnded, and InputChanged.

InputBegan does as you would think. It will trigger when a button is pushed.

InputEnded will trigger when a button is released.

InputChanged will trigger when a button has been changed.

An example script would be

local uis = game:GetService('UserInputService') --Get's the service.

uis.InputBegan:connect(function(key) --the defined variable, key, is the button pushed
    if key.KeyCode == Enum.KeyCode.Q then -- If the Key is Q
        print("Q has been pushed!")
    end
end)
uis.InputEnded:connect(function(key)
    if key.KeyCode == Enum.KeyCode.Q then
        print("Q has been released!")
    end
end)

ANIMATION

For animations, you must first define the humanoid, as you need to load the animation into the humanoid before you can play it. After you have defined the humanoid, you need to load the animation into it, which is commonly done when you're defining the animation. Then afterwards you need to play the animation.

local player = game.Players.LocalPlayer --Defines the local player
player.CharacterAdded:connect(function(character) --Waits for character to load, defines character
    local humanoid = character:WaitForChild('Humanoid') --Waits for humanoid to load
    local animation = humanoid:LoadAnimation(script.Parent.Animation) --Loads the animation into the humanoid
    animation:Play() --Plays the animation.
end)

COMBINED KNOWLEDGE

To roughly combine these, you would have the following script.

local player = game.Players.LocalPlayer
local uis = game:GetService("UserInputService")
player.CharacterAdded:connect(function(character)
    uis.InputBegan:connect(function(key)
        if key.KeyCode == Enum.KeyCode.R then
            local humanoid = character:WaitForChild('Humanoid') 
            local animation = humanoid:LoadAnimation(script.Parent.Animation) 
            animation:Play()
        end
    end)
end)

Keep in mind that this is not a complete script, and you would have to add this to your gun script, and make it more complex with the ammunition and the reload animation and all that.