Our group went forward with this Gestalt ruling in E6 format. So I'll share the experience so far, having held about 10 sessions so far.
Players started at 6th with 1 class, and each session grants a gestalt class level or feat.
A positive (imo) side effect has been that some encounters have been dealt with using more intelligent methods rather than just smash the exp/loot grab bag.
PC Builds
Half-Orc (Half-Bear
) Bear Totem Barbarian 6 // Champion Fighter 3 / Spell-less Ranger 3
Glaive with Sentinel & Polearm Master feats
Straightforward critical machine with some maneuvers. Rage's resistances sustaining effective hp through battle.
Half-Elf Vengeance Paladin 6 // Rogue 1 / Fiend & Blade Warlock 5
Houseruled Spell Blade invocation prior to Greenflame Blade's existence
Melee DPS with burst options. Tanks reasonably well between high AC and Warlock temp hp.
Aasimar Tempest Cleric 6 // Blue Dragon Sorcerer 6
Lightning Burst & some healing AC tank-mage.
Deep Gnome Rogue 1 / Mystic 5 // Illusionist Wizard 6
Brains of the party
High accuracy crossbow sniping, massive burst at times with Lethal Strikes
Utility spells & clever uses of illusions
Halfling Arcane Trickster Rogue 6 // Hunter Ranger 5 / Fighter 1
Skulking sniper. Consistent high damage and accuracy.
Dwarf Moon Druid 6 // Bear Barbarian 5 / Monk 1
Natural Attacks houseruled as monk weapons for purposes of Martial Arts.
Point man, high perception.
Favored form of Giant Constrictor Snake, combined with Rage makes for incredible EHPs and soak, as well as crowd control from constrict. Snek-Fu.
Leprechaun Bard 6 // ???
Leprechaun being custom and a blend of other races abilities
Player makes it about 1 session per quarter, usually causing comedic and erratic circumstances. Fitting player character.
We tend to have 4-5 PCs show each session. So far the party has survived all encounters, including some CR9 + minions. I'd estimate they can take on at least CR12. The synergies the party has built within their own characters, and with teamwork pays out really well. Enough melee feeding sneak attack conditions for ranged, advantage generation through constrict, temp hit points provisioned & refreshable, etc. Resource exhaustion & actual exhaustion condition have probably been the scariest things for them, as the more magic oriented classes rely heavily on those resourced abilities to stay effective.
The lack of complex high level magic I think has been fairly appreciated, keeping the turns fairly short and quick, rather than getting muddled in rules. The staple spells everyone loves are there. They haven't received any substantial magic treasure beyond a rare and a common as starting items.