Are there Gestalt or Hybrid rules for D&D 5e?
No, there are (currently, at least) no gestalt or hybrid rules for D&D 5e. The closest there is is the multiclassing rules.
It's difficult to prove a negative, but rules for gestalt or hybrid characters do not appear in any of the following sourcebooks:
- Player's Handbook
- Monster Manual
- Dungeon Master's Guide
- Elemental Evil Player's Companion
- Sword Coast Adventurer's Guide
- Volo's Guide to Monsters
Likewise, rules for gestalt or hybrid characters have not been featured in any of the Unearthed Arcana articles.
And, for the sake of completeness, they have also not appeared in any of the official campaign books.