How do I make a class selection GUI for a specific team?

Another alternative is to have the GUI only clone into players on the certain team whenever they spawn, for example:

game.Players.PlayerAdded:connect(function(p)
    p.CharacterAdded:connect(function(char)
        if p.TeamColor.Name == "Bright red" then -- If you just want Bright red players having it
            p:WaitForChild("PlayerGui")
            game.ServerStorage["GUINAME"]:clone().Parent = p.PlayerGui
        end
    end)
end)

You could put the script into StarterGui and hide everyrhing, then check the players team and if it matches then make the guy visible.

Something along the lines of this in a localscript.

Plr = game.Players.LocalPlayer

If Plr.TeamColor == BrickColor.new ("TEAM COLOR") then
-- make it visible. 
end