How to find all players in a server and store the names in a GUI?

My advice, use a local script to get the names of the Players.

Inside the localscript, put something along these lines:

local PlayerList = script.Parent.Parent.Parent.Parent --Adjust the amount of Parents for where the script is

function CreatePlayerlist()
    for a,r in pairs(script.Parent:GetChildren()) do --Clear old Names
        if r.ClassName ~= "Script" then
            r:Destroy()
        end
    end

    for i,v in pairs(PlayerList:GetChildren()) do --Adds names to Playerlist
        local PlayerButton = Instance.new("TextLabel",script.Parent) --Forgive my naming, super tired
        PlayerButton.Text = v.Name -- v is the actual object of Player, so access name property
        PlayerButton.Position = UDim2.new(1, -30, 0, (i-1)*16)
        PlayerButton.Size = UDim2.new(0, 30, 0, 16)
    end
end

Playerlist.ChildAdded:connect(CreatePlayerlist)
Playerlist.Removed:connect(CreatePlayerlist)
CreatePlayerlist() --Startup