a straightfoward translation of the Javascript version w/ closures should work

    function goal(n)
       return function(al)
           if al then
               return "G" .. string.rep("o", n) .. al
           else
               return goal(n+1)
           end
       end
    end

    G = goal(0)

    print( G()()('al') )

Also note that G()'al' is just syntactic sugar for G()('al') in Lua