Can you give me a screen shot of your code?
Instead of having so many ifs and elses, it helps just to make separate events.
event1 - when gasattack finishes "do X"
event2 - when bullet hits zombie and also gasattack animation is playing, "do x" then die (so that you don't lose the "do x" part on interruption.
Maybe I'm not understanding the problem correctly. But I was trying to do a lot of ifs and elses like you and it did become problematic. When I made them separate events for each possible scenario, the correct one always plays.