What could be the main cause of a Cylinder 1 misfire on a Pontiac g6 gt?

Replace coils and spark plugs should fix your problem 

The Ignition Module may be dying. If the easy stuff doesn't work, it's likely that.

Do a compression test and leak down test.

Those motors had a problem with the intake manifold leaking coolant. That could cause a misfire but in my opinion when the CEL flashes it's usually electrical.

Did you put AC Iridium spark plugs i the motor?

Bosch will cause misfires in GM motors.

With the money you already wasted on spark plugs wires & coil you could have paid a qualified mechanic to do a proper diagnosis and pinpoint the problem.

If your mechanic is suggesting you just randomly fix things without a diagnosis beyond the check engine codes then you should find a better mechanic.

Gas engines need 4 things to fire properly:

1. Air

2. Fuel

3. Compression

4. Spark (properly timed)

If any of those things aren't happening properly you'll have a misfire. Your car's computer doesn't know which part of that equation is missing, only that the cylinder is not firing properly. A decent mechanic would begin checking things like the spark, compression, fuel system etc and pinpoint the problem before telling you to just start replacing things.

You are "throwing parts", not the correct way to fix cars.  Troubleshoot properly.  Check for a fouled plug.  Check for spark with a spark tester.  Check if the injector is firing and check for injector pulse with a noid light if the injector is not firing.  Check for compression on #1 cylinder. 

Car is 23 years old. It will be not be a surprise if the #1 cylinder is shot.  Good luck.