Gestalt Warlock
Gestalt Rules from 3.5e follows the logic of having Class Levels in up to two or three other classes, but not actually counting as Levels in a Class for the purpose of Multiclassing. (To my knowledge).
Gestalt in 5e is no different, and follows the homebrew mechanics of this post in D&DWiki. There is no official rule for Gestalt Characters released by WotC, and the inclusion of anything from D&DWiki is often frowned upon, but the nature of specifically the Gestalt Warlock is as followed:
Warlock is often considered one of the strongest Classes to Multiclass into, along with Cleric or Rogue. As a rule of thumb, in higher roleplay or story-centric Campaigns, I dis-allow Cleric, Sorcerer, and Warlock multi-classes for the nature of how such a class occurs:
A character of true faith or personality steeped in religion can be Clerics. I've seen many Clerics handwaive the roleplay of their practice of religion or similar veins, even though the bare nature of being a Cleric or Priest means much more. In a vein not too similar to Warlocks, Clerics may get interaction or missions from their Deity, and it's entirely possible to lose power from your Deity if they no longer find you a valid champion of their beliefs.
A character must start with a Level in Sorcerer because the powers of one cannot just manifest so simply without prior innate magical powers. In a much more different vein to Warlocks, one may find abilities or powers to acquire a Gestalt Level in Sorcerer, such as being gifted a droplet of divine magic (Divine Soul) or gaining greater powers of draconic ancestry through dark immagical rituals (Draconic Ancestry).
A character must start with a Level in Warlock and the Pact scribed out in greater detail between Player & DM, and finding a Patron is something a character must find to Multiclass into. Or in this case, to become a Gestalt Warlock. Warlocks are defined by the fact they sought out or were acquired to a favorable or infavorable Deal or Pact with a being of greater power. In the case where a Character finds such a being and strikes such a deal for power (which rarely comes easy), the character will acquire a Gestalt Level in Warlock, ergo: Gestalt Warlock.
Gestalt Ruling
A Character with a Gestalt Level effectively gains the Class Features of the Level of Class they have acquired. What they do not acquire is:
- Hit Dice
- Any Class Skill or Saving Throw Proficiencies, much like regular Multiclassing
- Actual Levels that would go towards calculating a Character's Total Level (Such as Proficiency Bonus, Spell Slot Calculation)
In addition, if you have already have a Gestalt Level or Multiclass Level in one Class, you obviously cannot gain any more Levels into one or the other, nor would they both come to the same total Level. In the rare case Multiclassing is allowed.
Gestalt in Play
In play, a Player can either express their interest in becoming a Gestalt Warlock or there will be potential routes that would allow a Player Character to gain a Gestalt Level, but it will come from a choice that is not so easy to make.
Gestalt Warlock is NOT a Character Creation Feature
Gestalt Warlock is only going to come from if the Campaign already features Gestalt Rulings, such as if a Campaign only features 2-3 Player Characters who require a buff, or is an all-around super-hero esque higher powered PC Campaign.
Gestalt Warlock only exists as an example of the as-mentioned description for the feature: It is to allow a Character to make a diabolical, archaic, or antideluvian deal with a greater being of power, and not make them sweat about spending a Level or going through a great mock of effort to impart a boon on the Character.
Clarifying even further, this is a feature that only comes through during Campaign play, in which such a deal or promise can be struck in exchange for a proper Pact created with the Patron mentioned, which can be any of the aforementioned Patrons across Warlock Subclasses that you may find in your travels or adventures.
Imagine your party faces near death by the hands of rampaging demons and an archfiend hears the wails of your soul and is willing to trade it for the strength you need to defeat them.
Imagine you are helplessly divided and lost in foreign whimsical woodland and only the mighty archfae of the forest can guide you through at the cost of gifting it one of your favorite things in the world in exchange for learning how to travel the world properly.
Imagine the Great Cosmic Eye glares down upon your being, as weak and helpless as it is, and in exchange for piecing your mind back together it will replace one of your own eyes with its, so that you may spread its baleful glare upon the waking world.
And so on and so forth, ultimately you should ultimately imagine Warlocks will be the harbringer for coming narrowly to stirring or inciting great drama or undertaking natures that might bring them in conflict with the party, or might not. This is not always the case, but this is the real roleplay potential that comes from engaging with a higher entity that really isn't benevolent (cough Celestial Warlocks).
A character who wants to have the features of a Gestalt Warlock will not have them at the start of a Campaign from character creation, they should rather have an actual Warlock Character, as the purpose of Gestalt Warlock in the ruleset here is to describe what the actual feature might look like to surrender your Character to a higher power's whims in exchange for the features of one, and what that might look like.