How can I remove an item a player is holding?

Alright so first off, use variables. You typed that big GetPlayerFromCharacter line three times.

Keep your variable styles consistent -- capitalized, or not capitalized. You have Player, but then you also have hit and debounce.

Combine if statements with and.

Use the second argument of Instance.new to shorten your code.

Wait until everything is finished until enabling the script.

Don't put the space between the else and the if. What that's doing is basically just the same thing as what else does -- except with an if statement shoved inside. You can't have mutiple else's and you need an extra end. It's just a bad habit to get into.

So with these basic problems fixed (although it still won't work, because we haven't addressed the main issue yet), the script should look something like this:

local debounce = false

function onTouch(hit)
	local player = game.Players:GetPlayerFromCharacter(hit.Parent)

	if player and player.TeamColor == script.Parent.BrickColor and not debounce then
		debounce = true
		Instance.new("ForceField", player.Character)
		--Rest of code
		debounce = false
	end
end

Now for the main problem.

if Player.Backpack.Gun == Player.Backpack.Gun then

Alright, this just makes no sense. Of course the gun will equal the gun. This is one of the basic laws of logic. My cat isn't a parachute. My computer isn't a pizza. Something is what it is. A equals A.

But what if the gun is nil? Then you have a problem. You just indexed a nil value, and the code will error. You need to use FindFirstChild. Since it works like a function, instead of throwing an error if the child doesn't exist, it just returns nil, so it's useful in situations like these. Just keep in mind that it's only really useful in if statements.

local debounce = false

function onTouch(hit)
	local player = game.Players:GetPlayerFromCharacter(hit.Parent)

	if player and player.TeamColor == script.Parent.BrickColor and not debounce then
		debounce = true
		Instance.new("ForceField", player.Character)
		if player.Backpack:FindFirstChild("Gun") then
			--If it passed above, we know it exists.
			--Use Destroy, Remove is deprecated.
			player.Backpack.Gun:Destroy()
		elseif player.Character:FindFirstChild("Gun") then
			player.Character.Gun:Destroy()
		end
		wait(0.1)
		debounce = false
	end
end