How do I make when a player presses a button another gui pops up?

In future please post your script, even if it doesn't work, I assume by button you mean text button. Put this inside your text button:

local gui = PUT-THE-PATH-OF-THE-GUI-YOU-WANT-TO-OPEN-HERE -- (change this)
script.Parent.MouseButton1Click:Connect(function()
if gui.Visible == false then
gui.Visible = true
else
gui.Visible = false
end
end)