I don't know much about this emperor, or his coins, but I think most of the antoninianii of Gallienus are lighter in weight like doug said. Both your coin, and mine, are fairly early issues I believe, mine being struck in AD 258-9 and your's looks even earlier than mine, so that may have something to do with it..... I really don't know.
Just to make it worse: As with other successful coinages, imitations appear especially on the fringes of the area served by the 'real' coinage. These imitations are not really fakes and certainly not trash like the modern tourist items but should be separated from official issues and studied on their own. My example here is copied from a coin of Hormizd IV and has a Balkh mintmark but also has countermarks attributed to the Hephthalites who probably made the imitative host coin on which the countermarks were placed. We really don't know when the coin was made or when it was countermarked. What we know is that some ancient coins were made by governing officials imitating issues by other regions. There were Athenian looking owls made in Egypt, Roman looking bronzes made in Northern Europe or Gaul and probably many others that are not at all well understood. These coins are not fakes. Can we separate all of the imitations of Sasanian coins from far away mints from counterfeits made by ancient crooks in Sasanian regions from fakes made by modern crooks hoping to fool collectors? Not always but we try.