Desperate Gambit — ruling 1

The chosen source may be a permanent, a spell on the stack, or anything referred to by something on the stack (such as a Prodigal Pyromancer that’s left the battlefield while its ability is still on the stack)

Desperate Gambit — ruling 2

The coin is flipped during resolution of this spell, so you know in advance what will happen when the source deals damage the next time.

Desperate Gambit : règles, fonctionnement et interactions (MTG)

Desperate Gambit is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Choose a source you control and flip a coin. If you win the flip, the next time that source would deal damage this turn, it deals double that damage instead. If you lose the flip, the next time it would deal damage this turn, prevent that damage.

How it works : Desperate Gambit

Original English text — Wizards of the Coast / Scryfall

#1April 1, 2008· Wizards of the Coast
The chosen source may be a permanent, a spell on the stack, or anything referred to by something on the stack (such as a Prodigal Pyromancer that’s left the battlefield while its ability is still on the stack)
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
The coin is flipped during resolution of this spell, so you know in advance what will happen when the source deals damage the next time.
No other card is named in this ruling.