I have a 96 Dodge Neon.

Engine Check light came on. I had someone from Autozone check it using some sort of scan tool. Code said it is Cylinder #1 misfire.

After the scan test. Engine Check Light came off. Does it mean it is fixed or it was reset?

What could cause it to Cylnder #1 misfire? Spark Plug look OK.

Ever since this. It is eating a lot more gas.

Have you checked the spark plug gap? I had an identical problem with my 97 Accord that was corrected by shortening the gap. The condition of the plug wire is important also; older wires have higher resistance, so not as much power may be reaching the plug, also causing a misfire. Finally, is there a fuel odor on the plug? A strong smell of gasoline would confirm you're getting fuel to the cylinder, just no fire. Check engine lights don't reset themselves unless the problem was fixed, so I'll bet it was reset with the scan tool. Good luck.

OBDII vehicle engine control modules use a counter to measure misfires. If it doesn't see that count anymore, the ECM will ignor it and the light goes out. (but stored)
It could be anything simple from a bad plug, to a burnt valve. But something is definitely wrong in #1.
As I recall from working next to a Dodge dealership, that Neon's had many headgasket issues.