It does have GUI functions built in... but they kind of suck compared to other languages. If there's something missing that you really need, then you need to either make a userlib or else use a cheap replacement like this one.
Function skinwindow(window,backupimg)
skin=CreateCanvas(0,0,ClientWidth(window),ClientHeight(window),window)
SetGadgetLayout(skin,True,True,True,True)
skinimg=CopyImage(backupimg)
ResizeImage(skinimg,ClientWidth(skin),ClientHeight(skin))
SetBuffer(CanvasBuffer(skin))
DrawImage(skinimg,0,0)
FlipCanvas(skin)
Return skin
End Function
Function skinwindow(window,backupimg)
skin=CreateCanvas(0,0,ClientWidth(window),ClientHeight(window),window)
SetGadgetLayout(skin,True,True,True,True)
skinimg=CopyImage(backupimg)
ResizeImage(skinimg,ClientWidth(skin),ClientHeight(skin))
SetBuffer(CanvasBuffer(skin))
DrawImage(skinimg,0,0)
FlipCanvas(skin)
Return skin
End Function