Chevy truck please help?

[Lil B]

I have a 1997 chevy k1500. the check engine light is in limb mode. the whole truck shakes and rough idle. even rougher if you try to drive it. strong gas smell also. i replaced everything on this truck. If you name it its been replaced besides the body panels itself. Please help! code po300 comes up which is a random misfire in random cylinders. new spark plugs, wires, distributor, and it worked fine for 6 months and now this comes up. thanks in advance