Toyota went through 3 or 4 (5?) HG revisions over the life of the 3rd gen. Starting off with a mostly conventional one, adding larger steel surfaces, larger, and finally a completely steel "MLS' style gasket.
The problem, shared with many engines from many makes, is an aluminum head bolted to an iron block. When the engine warms up, the metal heat and expand at different rates. Causing them to rub slightly relative to each other. And the HG caught between them takes the brunt of this rubbing, and over time, it simply physically wears out. More of a matter of the number of heat cycles more than the overall mileage. So if they (somewhat) mythical JDM engine with 50K miles was used for lots and lots and lots of short trips, they could be closer to failing than one with 200K miles that did lots of long highway trips.
And of course, failure to maintain the cooling system with old coolant, the wrong coolant, or overheating it, can cause more serious damage.
My wife's '96 needed HG's when it was a bit over 300K miles. She just sort of rolled it in with a (more or less) due TB/WP replacement, since the front end of the motor needs to come off anyhow. That's half the work of getting the heads off.
With the heads off, the bores looked GREAT still, crosshatching still on maybe 95% of the bore. Just a couple of smooth triangles at the top of the bore. She used the latest revision HG's, just buy them for an 01/02 4Runner, they're all steel, and are best suited for the job of surviving between a block and head scooting against each other.