Here's some code.
local player = script.Parent.Parent.Player
local reason = script.Parent.Parent.Reason
script.Parent.MouseButton1Click:Connect(function()
local player = game.Players
local kick = player:FindFirstChild(player.Text)
kick:Kick(reason.Text)
end)
It wont kick the player!!

2 comments
its because you cant kick the player from the client (aka from a LocalScript) for security reasons, since exploiters can take advantage of this (kicking a player or the entire server out). so use a RemoteEvent to make the server do it
also I wouldn't recommend making a kick gui, as exploiters can take advantage of the communication and the arguments and kick everyone out of the server or a player. instead you could make a chat command since you probably wouldn't need any communication between the server and client