You might have missed a tooth on the timing belt and it is off timing a little bit or the camshaft sensor is off calibration from the air gap being disturbed.
I am going to give you a general diagnostic walkk through for throttle body back fires to eliminate some possibilites as the process of elimination is going to be the best way you wil have to find the problem. You might see something in the diagnostic that makes sense as being the problem after the head gasket was changed. Just us e the link below;

http://www.2carpros.com/articles/engine-backfires-while-running

Answered by DrCranknWrench (expert)
3,292 answers provided

when i unplug the #3 cylinder the popping goes away.

Response from Aaronmaster
1 question asked

Is that cylinder within the pressure differential specification..usually around 20% of highest cylinder pressure for the lowest pressure.

Answered by DrCranknWrench (expert)
3,292 answers provided