HG leak/failure is more prevalent in turbos but not limited to them. You have an aluminum head on an iron block, two different heat loading rates with the associated difference in expansion rates. The HG will eventually fail but on a N/A motor, it will probably be the slow process you are describing. As long as you have not overheated the engine, chances are the head is not warped & you can get away with just a new HG, not a difficult repair on a non-turbo, should take a Saturday if you never did one before, an afternoon if you have. You can check for head warpage with a couple of straightedges once it is off the engine.

If you want to see if there are other problems, have a leak-down test performed before disassembly. It will pinpoint trouble areas such as rings, valves, etc. Don't be surprised if something pops up with the mileage you have.

If you are willing to roll the dice, you could just fill the head cavities (intake/exhaust ports) with water once the head is off the engine to see if any valves are leaking. Quick & dirty but will show up anything bad.
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1999 E320-200K 1994 F150 (awaiting a new engine) 1989 560 SL-180K 1994 945T (daughter's)-312K 1986 242Ti (awaiting a B230FT transplant) 1968 GT500KR (under slow restoration) 2007 HD Nightster (son's ride)