Time seems to have corrupted any clear distinction, and things are most likely different in England and other places. Unfortunately there are so many different standards that using gage to specify a material is almost useless, and perhaps even dangerous. If in doubt, use decimals. For example, in U.S. gage, the standard for sheet metal is based on the weight of the metal, not on the thickness.
Smaller numbers refer to greater thickness.
There is no formula for converting gage to thickness or weight. If you need specific values for specific gages, many are listed in various references such as Machinery's Handbook .