SCLSGS400

I have a LS with 310,000 miles that I do not DD, it was running great up until recently.
I noticed when I took it for a drive and hit 60mph the car would feel a sudden loss of power and when I tried to accelerate it sort of jerked for a bit and the CEL came on.
I scanned it and it read p0306 cylinder 6. So I replaced all the plugs and wires and during so I found oil in the plug well and my intake had brown build up before the throttle (I cleaned it).
The CEL went away, I took it for a test drive, it ran great but I was still getting the issue at 60mph but no CEL.
I thought it might be bad gas since the car has been sitting however when I made it back home I smelled burning oil and there was smoke coming from behind my engine.

Could this be a bad head gasket giving me these issues i feel at highway speeds? Or maybe a bad throttle? Trans solenoids?

I've had this car for over a decade and the body and interior are in great shape, id love to fix this if I can.


Thank you