How would you go about replacing a char/word in gsub?

First of all, I am not that great at manipulating strings. I recently started to make my own chat gui which was not that hard to make but with a filter feature. Recently, I have been in a bit of a trouble trying to replace chars/words. It seems that I am in the right path but this error calling 'gsub' on bad self (string expected, got table) is bugging me right now.

local chatGui = script:FindFirstChild("ChatGui") or script:WaitForChild("ChatGui")
local filter = { "hello"; "lua"; "noob"; "subjective"}

local replacement_words = {"apple", "ambiguous", "banana", "bit", "byte", "car", "clock", "cold", "front", "game", "gold",  "world", "pizza", "door"}

game.Players.PlayerAdded:connect(function(player)
	local cloneit = chatGui:Clone()
	cloneit.Parent = player.PlayerGui
	player.Chatted:connect(function(msg)
		print(player.Name .. " has said: \""..msg .. "\"")
		for word in msg:gmatch("%S+") do --This first loop splits every word the user has said
			for _, v in pairs(filter) do
				if string.lower(word) == string.lower(v) and string.len(word) == string.len(v) then --Checks every word and the correct length of that filtered word
					filtered_word = v
					print("Filtered word got: \"" .. filtered_word .. "\"")
					local replacement = tostring(replacement_words[math.random(1, #replacement_words)]) --Here is the main issue
					print("Replacement word is: \"" .. replacement .. "\"")
					print("Found filtered word in our filter system. Replacing that word")
					new_S = string:gsub(msg, v, replacement) --calling 'gsub' on bad self (string expected, got table)
					print(new_S)
				end
			end
		end
	end)
end)