No Compression

Hello,
I have been trying to get a slant six that has not started in 13yrs from last time registered 99' it has spark and fuel but no compression. I got O readings from three out of the four cylinders and 30 psi out of #6 stop checking after that. So my question is if its not reading anything it must be that the head gasket is gone right? I would think that piston rings wouldn't be worn that bad what do you guys think?

denis