Horwath
Legend
I honestly think that if you are trying to replicate the "old school feel" of combo-type-classes, like Elf (Fighter / Mage) or Rogue / Fighter, etc... then the standard 5e rules (i.e. multiclassing) are more than sufficient, you don't need to try and create a Gestalt option to cover that. A Gestalt PC, with standard PC's, is just asking for problems at the table, which are not worth the trouble of trying to fix and balance.
For example, if you want to be a "fighting wizard", you can be an Eldritch Knight, or simply fit in a couple of levels of Fighter, or do both, and you're good to go. Sure, the first couple of levels might not quite fit the archetype you have in mind, but that's how the 5e rules are well balanced as-is; by the time you are say level 3+, you, like 'straight' classes, are all settling in to your chosen archetype. Same for "clerical rogue", etc etc.
If you want to be a "fighting rogue", just decide what you want more of (sneak attack, or fighter stuff), then take most levels in that class and just 2-3 levels of the other class.
Same if you want to cover divine and arcane magic; multiclassing works pretty well here (you have to choose 'one main path'; allowing one PC to have access to virtually all the spells makes the rest of the party pretty redundant).
I agree with you, but gestalt is not making the rest of the party redundant. It is about filling in core roles of the party with few people available. I.E. 3players. You need; fighter,rogue,cleric,wizard(or classes that fill that role equally).
Players sometimes want to play cool-not so effiecient characters, then if you do not have 5-6 size party, your party cohesion suffers. 3 players gestalt offers abilities from 6 classes, so you are most likely be sure to get 4 core roles filled, with 2 extra flavor combinations.