Engine runs rough/hesitates and feels like I'm forcing it when speed up. on 1998 Infiniti I30

Check engine light is on (sometimes flashes). I just replaced a damaged fuel injector- gas was coming out of tailpipe. When I replaced spark plugs some time ago, was told there was oil in the sockets. Was also told that it could be head gasket. What are the symptoms for that and approx. cost for repairs? Would I have to replace all engine gaskets?

Asked by for the 1998 Infiniti I30
Siunds like u need good tuneup with vavle cover gasket...