Okay so, it works like... Classes:
You can combine two classes, but not two variants of the same class (So for pathfinder, no Skirmisher Ranger / Divine Tracker Ranger, or Cavalier / Samurai combos) Hit Dice:
Choose the higher of the two classes' HD. A Barbarian / Sorcerer uses D12, for instance. Saves:
For each save bonus, choose the better progression between the two classes. So a Fighter / Wizard gestalt gets the good Fortitude progression of the Fighter, the good Will progression of the Wizard, and the poor Reflex progression of both. This is the progression itself, not the bonuses, so it doesn't "stack" - a Ranger / Monk 1 has a Save lineup of F+2 / R+2 / W+2, not F+4 / R+4 / W+2. Base Attack Bonus:
Choose the better progression of either class. Skills:
You get the higher number of skill points between the two classes, and treat all skills from both classes as "Class skills." Class Features:
You just combine them; Our 1st-level Ranger / Monk gets Favored Enemy, Track, Wild Empathy, a Monk bonus feat, flurry of blows, stunning fist, and unarmed strike.
If both classes provide a feature at different levels, you get the earlier entry first. A Barbarian / Rogue gets Uncanny Dodge at level 2, instead of level 4
Class restrictions still apply; a Paladin / Anything gestalt must be Lawful Good, for instance, a cavalier / druid can't wear metal armor, and a fighter / wizard has to contend with arcane spell failure. *Pathfinder Special*:
Some Pathfinder classes have abilities that are similar but different - a good example is the cavalier Mount and the druid animal companion. Under Pathfinder multiclassing rules, a Druid / Cavalier multiclass character can either have a creature from the Cavalier mount list as his druid companion, progressing it with his full levels, or
he can take both features, and have both a mount and a companion, though both are weaker due to splitting his levels between the two. I would suggest that for a gestalt, conflicts like this default to the simplest solution; the Cavalier / Druid gets a druid companion that can work like a Cavalier mount. Congratulations, you're riding a T-rex.Spellcasting:
Multiclassing and Prestige Classes:
The "basic" rules say that you track spell slots separately, while the examples (Cleric / Sorcerer, Wizard / Sorcerer) say it works like Mystic Theurge "On Steroids"
On the one hand, you could have a Sorceracle gestalt, and it knows 4 level-0 oracle spells, four level-0 Sorcerer spells, two level-1 oracle spells and two level-1 Sorcerer spells. Assuming 18 charisma, it has 4 Spontaneous slots just for oracle spells, and 4 spontaneous spell slots just for sorcerer spells.
On the other hand, the Sorceracle knows all those spells, but can mix-and-match as they please; maybe they want eight sorcerer spells today. Or six oracle spells and two Sorcerer spells.
or... "kind of both" - The Druid / Cleric has Two Druid slots, two cleric slots, one domain lot. he can prepare four instances of Magic Fang if he likes (that domain slot is domain-only) but can only use his two druid slots for Summon Nature's ally - the same with Cure spells and his Cleric slots. or... the spontaneous / prepared version of that, a Witcherer has a familiar that contains all the level 0 Witch spells, plus 3+Int level 1 Witch spells. He also knows four Sorcerer/Wizard level-0 spells, and two level-1 Wizard/Sorcerer spells. He has 4 prepared level-0 slots, two level-1 prepared slots, and two spontaneous level-1 slots. He can prepare any combination of his Sorcerer and Witch spells, and can spontaneously cast any spell he Knows or has prepared, with his spontaneous slots. If a Wizard / Witch loses their wizard familiar, they can only prepare Wizard spells; if the familiar ran off with their spellbook before getting killed, they're stuck with what they had prepared before that disaster. That sort of thing.
since in pathfinder, there are only a few classes that wholly lack spellcasting ability (Barbarian, Brawler, Cavalier, Fighter, Gunslinger, Swashbuckler) I think it's definitely need-to-know
You can multiclass with another gestalt combination or a single class, if you prefer. Prestige classes can also be used as gestalts (though again, their requiements and restrictions remain.) *Pathfinder Bonus*
: Favored Class Bonuses; Want to just say that you pick a gestalt class as your favored class, and gain one of the following: +2 hit Points, +2 skill points, +1 HP and +1 Skill point, +1 HP and one alternate FCB, +1 Skill point and one alternate FCB, or the FCB of both classes in your favored gestalt?