i have 1999 malibu 3.1 v6 when i changed spark plugs i seen gas spraying from fuel rail then it caut on fire what is the problem that caused this

The rubber gaskets or seals are worn. When you disturbed them, what ever was keeping the gas in was broken and caused the gas to leak out. The entire system needs to be re-plumbed with new gaskets and seals. If the system is leaking after 5 years of service, then have it completely gone through. Also check to make sure all the connections are secure and tight. Almost all gasoline sold on the US market is good gas and should not contribute to the rapid deterioration of the gaskets. Do you live near the ocean where salt moisture could cause premature failure? Very cold weather, like last/this year can make them brittle and start small breaks in the gaskets or seals. Have they been removed and replaced and not seated properly? Have you had them cleaned out or used a gum/residue-out cleaner that may help in the eroding process? Get it changed out or make sure your insurance is paid up.