How strong it is depends on how well the classes synergize.
For example, if someone wanted a Gestalt Archivist (INT based divine caster) / Wizard that'd be worth +3 to +4 LA due to the massive versatility and stackable class abilities. Ditto for Favored Soul / Sorcerer.
If, otoh, someone wanted to a Gestalt Barbarian / Ranger, I'd consider that to be worth only about +1 to +2 LA, because there's a lot of overlap between the two classes.
Similarly, given the stacking, Fighter / almost anything should be worth around +3.
On the other other hand, if a PC wanted to Gestalt with NPC classes, that'd be worth about an LA less. For example, something like Rogue/Warrior or Fighter/Adept would probably be only worth +1 LA at low levels to mabye +2 LA at high levels.
The exception among NPC classes being Expert, which is potentially worth +2 LA because it makes qualifing for PrC's so much easier.