1998 Honda Civic Poorer gas mileage over last six months?

Up until 6 months ago, the car was regularly maintained. I haven't driven this car much in last six months because I go to school now. Which means very less maintenance. Oil and filter changes are regular.

The spark plugs were replaced at 55k, distributor and exhaust system is also brand new (5k miles ago). The car runs fantastic otherwise.

Over last 5-6 months (in cold cold city), it is continuously performing poorer on gas mileage. From a sweet 32 MPG to a poor 20 MPG :(

Today, I took it to a repair person (who sounded trustworthy) - he first thought (since we smelled gas from the outlet on the top of the engine when it took the cap off, and obviously with a lot of air) that it could be bad head gasket spoiling the compression. He took opinion from another mechanic and said that the head gasket should be OK and it could be the valve or piston and it could be costly to open (and grind n stuff) and figure out if we fixed it.

After he thought for a while, he said, let's start with basics. He cleaned the fuel system and checked the fuel pressure again and he said that it went up from 30 to 40 (psi?) And we could smell no gas from the engine. The engine is running smoother now.

The car ALWAYS runs cool (never overheated), no leakages (full level of coolant/blue antifreeze). No foul smells in the cabin, and clean air filter. Do you think it is the head gasket spoiling the gas mileage, or we just fixed the problem by simply cleaning the fuel system which wasn't done for last 15k miles?

I will be taking some long drives in next couple of days to see if there is any difference or I go back to him. In the meantime, please suggest! Thanks.