I wasn't saying that any of the party members using or not using gestalt had to be casters. But you are right its the high level casting that shows where the real differences lie.
By the way Surreal I did some calculations on the bonus XP from being lower level but treating both sides of the gestalt as needing XP. It was scary. Depending on the encounters faced and the rest of the party (the most extreme example is a large party of non gestalts with only 1 gestalt character) the bonus XP could be insane.
For a crazy DM (like me) who plays only 'overpowering' Difficulty ratings the xp above the rest of the party would be around +25%. However if the DM gave easier encounters the xp % could skyrocket, enough to allow the gestalt character to almost catch up to the party (or even worse, the gestalt character could choose to stay more than a level down and item craft).
For a normal DM I don't see this being good... When I DM I don't allow extra XP from holding back a level (not leveling up) or by using up XP (either from crafting or spells).
But unless anyone has any reason to question these general rules (yes eyeballing is always good for specific DMs), this is what I will take away from this discussion:
1) the LA and HD matter needs to be cleared up. For those following the inane but RAW ECL=HD+Classes: LA should be on one or both sides but be applied alongside or before the LA. For 1 LA side users, if an LA is applied a class level should NOT be present on the other side. An optional exception would be for Racial LA or racial HD for an even more power.
I prefer my ECL variation for mostly-free racial HD & having the LA on 1 side & allowing class levels on the other side of Racial LA. (For those who like low power campaigns LA should be required on both sides and the regular ECL calculations will ensure everyone is human)
2) The pure-combatant vs hybrid vs full caster arguments seems like an excuse to keep casters the ROFL PWNZORZ that they are in non-gestalt. I like balance (no really!) so even within a specific tier empowering the gishes and pure combatants while not really helping the casters seems fair to me. If anything, they are the danger. I even question if I'd allow the full casters to go the full 20 class level non-gestalt manner. They will still be stronger than the gish or pure-combatant anyways even when hampering them down to a max for 15//15. This leads to the class levels.
3) I'd say that a slightly underpowered mapping would be this: 5=4//4, 10=8//8, 15=12//12, 20=16//16,
Regular Gestalt
1 1//1
2 2//2
3 3//2
4 4//3
5 4//4
6 5//5
7 6//5
8 7//6
9 7//7
10 8//8
11 9//8
12 10//9
13 10//10
14 11//11
15 12//12
*16 13//13*
17 14//13
18 15//14
19 15//15
20 16//16
*Here is the sweet spot before regular old wizards start throwing around 9th level spells and everything normally goes to Hell.