A choice of quick cheats for those that don't have time to keep referring to the ESP8266 GPIO pin -> IO index map.

shim-global.lua

-- You could setup global vars:
for k,v in ipairs{3,10,4,9,2,1,nil,nil,nil,11,12,nil,6,7,5,8,0} do _G['GPIO'..k-1]=v end

-- and use them like this:
gpio.mode(GPIO2, gpio.OUTPUT)
gpio.write(GPIO2, gpio.WRITE)

shim-namespaced.lua

-- Or set them up in the gpio namespace:
gpio=setmetatable({},{__index=gpio}) for k,v in ipairs{3,10,4,9,2,1,nil,nil,nil,11,12,nil,6,7,5,8,0} do gpio['GPIO'..k-1]=v end

-- and use them like this:
gpio.mode(gpio.GPIO2, gpio.OUTPUT)
gpio.write(gpio.GPIO2, gpio.WRITE)

ipairs is not working for me. The loop stops at first nil value.

Changed ipairs to pairs and worked ok.