The term "gestalt" is used in our context to describe the effect of having multiple spellcasting classes working together in combination for a character. The saying that "the sum of the parts are other than the whole" actually implies that a gestalt or multiclassed spellcaster would not be more powerful than a "pure spellcaster" of the same level, but would instead be something altogether different.
Based on this concept which is loosely rooted in the variant rules presented by the Unearthed Arcana supplement, characters with more relevant spellcasting classes will have a greater advantage and be "full gestalts" whereas characters with less similar spellcasting classes will be "half gestalt."
In this paradigm, a bard/wizard 4/4 (a full gestalt) has reasonably achieved a similar mastery of the Weave to a character with simply one of those classes; her arcane casting classes both use an effective Arcane Caster Level of 8. Although this does not affect her number of spells available per day, both her classes benefit from the increased Caster Level as would effect duration damage, or similar effects for her spells.
A character with more disparate classes such as a bard/druid 4/4 (a half gestalt) has achieved a somewhat lesser mastery of the Weave than a character with simply one of those classes, but they cover a wider spectrum of abilities. Such a character has her Arcane and Divine caster level set to 6, as she's adding half the secondary casting class.
The following classes gestalt for divine casting purposes:
Blackcoat | Blackguard | Blighter | Cleric | Druid | Paladin | Ranger | Shadow Adept
The following classes gestalt for arcane casting purposes:
Assassin | Bard | Bladesinger | Eldritch Knight | Harper Scout | Pale Master | Shadow Adept | Sorcerer | Spellfire Channeler | Warlock | Wizard