Formal:
* Gibbet - This is the most direct synonym, referring specifically to the structure used for public execution by hanging.
* Scaffold - This word can also refer to the platform used for executions, but it can also refer to other platforms, like those used for construction.
* Hangman's tree - This is a more poetic and specific term, referring to a tree used for hanging.
Figurative:
* Doom - This refers to the impending fate of death, associated with the gallows.
* Fate - This is a broader term for the inevitable, which can be associated with the gallows.
* Pit of despair - This is a metaphor for a hopeless situation, often associated with the feeling of impending doom.
Informal:
* Hangman's noose - This refers specifically to the rope used for hanging.
* The drop - This is a slang term for the execution itself.
The best synonym for you will depend on the specific context and tone you're aiming for.