Anyone known how I could make a text label in a gui change to a players username in the gui?

This is actually quite simple. Here's how:

-- This is how you would do it in a LocalScript

local Player = game.Players.LocalPlayer -- Defining what the Player is 
local TextLabel = script.Parent -- Assuming you parented the script to a TextLabel

TextLabel.Text = Player.Name -- Changing the TextLabels Text to the Local Player's name




-- This is how you would do it in a Server Script

game.Players.PlayerAdded:Connect(function(player) -- defining the Player
    local TextLabel = script.Parent -- Change this to the path of the TextLabel
    TextLabel.Text = player.Name -- Changing the TextLabels Text to the Player's name
end)

Please accept this answer if this helps!

while wait() do
    script.Parent.Text = ""..game.Players.LocalPlayer.Name
end