If a Pokemon has a poison condition that places more than one damage counter per turn, like from Crobat's "Ultra-Toxic Fang" attack, does Virbank City Gym work or not?
Yes, it works. You place 4 damage counters for Crobat's "Ultra-Toxic Fang", then you place 2 more for Virbank City Gym making a total of 6 counters placed.
Source: TPCi Rules Team (2013-02-28)

Relevant cards

Note: All applicable cards may not be listed below.
BW8_126