After replacing the head gaskets engine will not idle

Could very well be the old gas, especially if you are in a place the has ethanol in the gas. I would probably disconnect the fuel line and connect an extension to it and use the pump to remove as much of the old gas into a couple portable cans as you can. Then once it stops pumping dump in some ethanol free gas and run the pump a few seconds to get that into the lines. Reconnect the line to the engine and let it run a bit.
To use the pump that way you connect battery power to the pumps prime connection, It is a Gray wire that is usually right around the fuse box or PCM on the GMs. Battery power there will turn the pump on full time. If new fresh fuel works then I would add some stabilizer to the next 5 gallons, especially if it will be a while until you will be driving it a lot. Most of the gas sold in the past few years with ethanol added is garbage and really only stays close to usable for about 90 days. If you have another vehicle you can slowly add the stuff you pumped out to it's tank, like a gallon into the tank and top with new gas. That way you can use it up instead of trying to dispose of it. I would probably dump some into a clear glass and let it set a couple hours and see what settles out first though.

I would start with fresh fuel to eliminate it first. If this was a daily driver that hadn't sat around we would start elsewhere but bad fuel can toss you for a loop.

In that case good luck and please keep us in the loop. What is it doing when it won't idle? Just shutting down or runs real rough?