[3.5] CR of Gestalt NPCs

[SlimeKnight40]

I know that the CR of an NPC would be equal to the number of (player) class levels they have (Fighter 6/Monk 4 would be CR 10.) I'm not sure how to rank the CR of a Gestalt character (like a Fighter // Monk 10). I would guess that the CR might be adjusted by +1 from Unearthed Arcana's suggestion that enemy CRs could be reduced by 1, but that seems like it may not be appropriate.

In a similar vein, would using Gestalt NPCs for boss fights be an inherently bad idea in an otherwise non-Gestalt campaign?

[Vhaidara]

Gestalt is only powerful if you do it right (a passive side and an active side) or if you abuse the action economy (factotum).

For example, gestalting fighter onto wizard give you hp, saves, bab, and feats. But, if you're casting, as a good wizard should be, those are mostly irrelevant.

A more obvious examppe is that everyone is gestalt. Just that the other side is commoner. Which means no benefit.

[ksbsnowowl]

In a similar vein, would using Gestalt NPCs for boss fights be an inherently bad idea in an otherwise non-Gestalt campaign?

Very bad idea that your players would (likely) rightly be mad about.

I'll answer your first question in a roundabout way:
In my gestalt game, if the PC's are all 10th level (Rogue 10//Wiz 10, etc), then a 10th level Gestalt foe is CR 10 and a non-gestalt 10th level foe is CR 9.
(I often note their Gestalt CR, or GCR beside their normal CR value, so the non-gestalt 10th level foe is CR 10 [GCR9].)

[Gnaeus]

I know that the CR of an NPC would be equal to the number of (player) class levels they have (Fighter 6/Monk 4 would be CR 10.) I'm not sure how to rank the CR of a Gestalt character (like a Fighter // Monk 10). I would guess that the CR might be adjusted by +1 from Unearthed Arcana's suggestion that enemy CRs could be reduced by 1, but that seems like it may not be appropriate.

In a similar vein, would using Gestalt NPCs for boss fights be an inherently bad idea in an otherwise non-Gestalt campaign?

Remember that the Ftr 6/Mnk4 IS CR 10, but is barely a challenge for an ECL 6 party, far weaker than most ECL 10 monsters, while a Wizard 10 is also CR 10 but is a very difficult encounter. Gestalt just pushes that spread farther.