This line:
skingui = guiCreateWindow(500,200,290,440,"Skin Shop",true)
The values are set as Absolute, but you set it as Relative ( 'true' at the end ).
Should be:
skingui = guiCreateWindow(500,200,290,440,"Skin Shop",false)
local item = guiGridListAddRow ( grid )
guiGridListSetItemText ( grid, item, columna, getPlayerName (unpack( alivePlayers )), false, false )
Eso no tiene el menor sentido.
Mira el ejemplo aca:
https://wiki.multitheftauto.com/wiki/GuiGridListAddRow