If you have a pair of micrometer calipers handy (or a micrometer, if you have one) you can measure the wire diameter and then determine your wire gauge with the help of this conversion table. The table lists the size and corresponding gauge of the conductor only and not of the overall wire diameter.
Recently I was in similar traouble. I ordered carparking gate made of GI sheet. The vendor fellow told to provide 16gauge but how to confirm? I purchases a gauge plate containing facility to measure plate and wire size. It costs hardlt 2 US$ (Rs 100) but is great during need. Try this!!