Starting Level: 1st Level Gestalt
Ability Score Generation: 16,15,14,13,12,11
Alignment: Any Good, maybe Neutrals. No Evil Characters.
Starting wealth higher of your two classes.
Maximum HPs for each level.
No multiclassing.
Setting will be in the Forgotten Realms.
Each character starts out with 1 bonus feat (Variant Humans not allowed in the campaign)

Books I own that you can draw from:

Players Handbook
Dungeon Masters Guide (Probably)
Sword Coast Adventurer
Xanathar's Guide to Everything
Tasha's Cauldron of Everything (Probably)
Eberron: Rising from the Last War (Maybe)

How I view Gestalt in 5th Edition to work:
Hit Dice: Larger of your two classes at each level.
Armor Proficiencies: Combine your armor proficiency lists.
Weapon Proficiencies: Combine your weapon proficiency lists.
Tools: Combine your tool list for proficiency.
Saving Throw Proficiencies: Combine your lists, select two.
Skills: Combine your skill lists, select the higher choice option from your classes. (Rogue gets 4, Barbarian gets 2, so a Barbarian/Rogue would get 4 choices from the combined list)
Equipment: Select from one of your two choices for each section (A,B,C).
Spells: Spells Known and Spells per day each have their own separate list. Spells cast must be used from the proper class spell slots. If there is an ability that affects a character's recovery of spells, it functions only for the class that grants it.
Ability Score Improvement: You get this only once for every 4 character levels. You may select a +2 to an ability score, two +1 to different scores, or sacrifice the ability score improvement for a feat.
Extra Attacks: Do not stack, you take the better of the two in your class progressions.