guiGetScreenSize helps you to get the resolution of the player, and you cand make the math like this: sW,sH = guiGetScreenSize()
guiCreateWindow(sW*0.4,sH*0.4, 200, 200, "...", false)
and it is the same thing as using the relative version
guiCreateWindow(0.4,0.4, 0.3, 04, "...", true) **but you need to use values between 0 and 1 everywhere**