Gargantos, the Endbringer

Doubling a creature’s power and/or toughness creates a continuous effect. This effect modifies that creature’s power and/or toughness but doesn’t set those characteristics to a specific value. See rule 613.4c.

To double a creature’s power, that creature gets +X/+0, where X is that creature’s power as the spell or ability that doubles its power resolves. Similarly, an effect that doubles a creature’s toughness gives it +0/+X, where X is that creature’s toughness. Doubling a creature’s power and toughness gives it +X/+Y, where X is its power and Y is its toughness.

If a creature’s power is less than 0 when it’s doubled, doubling that creature’s power instead means that the creature gets -X/-0, where X is the difference between 0 and its power. Similarly, if its toughness is less than 0 when doubled, it gets -0/-X. If one characteristic’s value is negative but the other isn’t when both are doubled, it gets -X/+Y or +X/-Y, as appropriate.

To double a player’s life total, the player gains or loses an amount of life such that their new life total is twice its current value.

To double the number of a kind of counters on a player or permanent, give that player or permanent as many of those counters as that player or permanent already has.

To double the amount of a type of mana in a player’s mana pool, that player adds an amount of mana of that type equal to the amount they already have.

To double an amount of damage a source would deal, that source instead deals twice that much damage. This is a replacement effect.

Deathtouch is a static ability.

A creature with toughness greater than 0 that’s been dealt damage by a source with deathtouch since the last time state-based actions were checked is destroyed as a state-based action. See rule 704.

Any nonzero amount of combat damage assigned to a creature by a source with deathtouch is considered to be lethal damage for the purposes of determining if a proposed combat damage assignment is valid, regardless of that creature’s toughness. See rules 510.1c–d.

The deathtouch rules function no matter what zone an object with deathtouch deals damage from.

If an object changes zones before an effect causes it to deal damage, its last known information is used to determine whether it had deathtouch.

Multiple instances of deathtouch on the same object are redundant.