The wine does have the last say, but the three main factors that I see (after vintage) are:
#1 Terroir: If you look at the three top villages in the Pfälzer Mittelhaardt (Forst, Wachenheim, and Deidesheim), you will quickly see that not all GGs are created equal. The Deidesheim Crus tend to be primarily sandstone (Buntsandstein) and are lighter with more citrus fruit, and snappy, racier acidity. The aromas are friendly and very forward. Sandstone warms up quickly and has excellent drainage properties, keeping the berries small (but also limiting available nutrients), and the grapes have rather thin skins, making wines with potentially less complexity and staying power. Though they tend to be the most impressive when young.
The heavier Limestone soils (Muschelkalk) of Wachenheim help to buffer acidity but make wines that taste more acidic, perhaps partially because the wines suggest more mineral, rocky, salty flavors, (the most mineral in the region for many tasters). These wines can be very long-lived, very complex because of the fruit/mineral tension, and also the most chiseled or defined.
Forst, with all of its volcanic Basalt soils makes some of the most complete and exotically flavored wines in Germany. Spices (cinnamon, ginger, allspice), stone-fruit, passion-fruit, and outrageous floral aromas are the hall-mark of Riesling grown on Basalt. The vines want for nothing, as Basalt is extremely mineral-rich and (relatively) quickly breaks down into a clay that holds water and nutrients exceptionally. The grape-skins are thick (protecting against botrytis in most years, but the available water can cause bursting in very wet years). As you move north from the Kirchenstück (which has the highest proportion of Basalt) towards Wachenheim, the soils start to mix with Muschelkalk and sandstone, producing wines with greater-degrees of finesse (first Jesuitengarten and then Pechstein, which is a true Forst/Wachenheim hybrid and many peoples favorite because of that.)
#2 Vine age: Most producers in the Pfalz won’t produce a GG from a vineyard, regardless of its fame, until the vines are at least 15 years old. Wachenheim was one of the first villages in Germany to undergo Flürbereinigung, which is why Bürklin-Wolf with all of its holdings there has some of the oldest vineyards in the region (the exception is Wachenheimer Böhlig, which is newly planted).
#3 Cellar techniques. There are frankly too many different variations and philosophies here to list, but if we very generally take the 3B’s (Bürklin-Wolf, Bassermann-Jordan, and von Buhl), all of whom have parcels in the greatest vineyards of the Mittelhaardt, it is interesting to see why the wines taste different. Von Buhl is generally considered to make the most forward,fruity, easy drinking-style of the three. For the most part, the grapes are whole-cluster pressed and fermented using cultured yeasts. Bassermann does some spontaneous fermentation in both steel and old wood (sometimes new wood), farms Biodynamically, and sometimes adds a proportion of sweeter wine to its GGs (Auslese, BA) to round out the wines and add texture while walking the trocken-line. I for one, really like most of their top wines (and not-so-top wines) and think that they age beautifully. Bürklin-Wolf is the most traditional of the three. Sometimes whole-cluster, but most always spontan-fermentation in large, old oak casks –bottled later than the rest, lots of lees-contact. BW undeniably makes some of the longest-lived dry wines in Germany and also among the most complex. Also Bio-dyn.
Overall, I’d personally say that healthy grapes, some skin maceration, must-oxidation, cask fermentation, long-lees contact, and minimal filtration are a good start after the important requirements of terroir and yield are accounted for.
I have no doubt that the von Winning wines (as polarizing as they may be in their youth) will age into spectacular wines, and I’ll echo what others have said about Koehler-Ruprecht and Rebholz being long-haulers. I like the Christmann Idig a lot too (it is a different, much better wine than even a few years ago). 2009, 2010, and 2011 are masterpieces in waiting.
Cheers,
Bill