I have never come across a fake Gallienus bronze before. The genuine coins are just too common and cheap to make it worthwhile.
Interesting side note is that while 99% of the Gallienus coins encountered are from the later hyperinflation and all have the same portrait, yours is an older coin using a younger portrait. I have one that is actually (more than 20%) silver, and I couldn't even recognize him until I made out the legend.