What is the Enum for Holding down the left mouse (UserInputService)?

With the UserInputService you can use InputBegan and InputEnded to keep track of automatic firing.

local firing = false

UserInputService.InputBegan:Connect(function(input, gameEvent)
    if not gameEvent then
        if input.UserInputType == Enum.UserInputType.MouseButton1 then
            firing = true
        end
    end
end)

UserInputService.InputEnded:Connect(function(input, gameEvent)
    if not gameEvent then
        if input.UserInputType == Enum.UserInputType.MouseButton1 then
            firing = false
        end
    end
end)

I don't think there is a way to know, but there is another way to do. We can check if the player is holding the mouse with tool.Activated and tool.Deactivated to a boolvalue:

local tool = script.Parent > the script's parent is the tool
local HodingMouse = script.Parent.HoldingMouse >  you have to insert a boolvalue named "HoldingMouse" in the the tool so the script can detect automatically after the holdingmouse's value becomes true instead of doing a "while wait(.5)"

tool.Activated:Connect(function()
    HoldingMouse.Value = true
end)

tool.Activated:Connect(function()
    HoldingMouse.Value = false
end)

script.Parent.HoldingMouse.Changed:Connect(function()
    if script.Parent.HoldingMouse.Value == true then
        repeat
        > FIRES THE GUN
        wait(.5) > THE GUN HAS TO WAIT A BIT BEFORE RESHOOTING
        until script.Parent.HoldingMouse.Value == false > STOPS THE GUN WHEN THE PLAYER STOPPED HOLDING
    end
end)

I hope this helped!

There's no direct Enum for holding the mouse but there is a trick you can use to detect it.

Example

local userInputService = game:GetService("UserInputService")
local holdingMouse = fasle

userInputService.InputBegan:Connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseButton1 then
        holdingMouse = true
    end
end)

userInputService.InputEnded:Connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseButton1 then
        holdingMouse = false
    end
end)