Gargos, Vicious Watcher — ruling 1

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Gargos). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

Gargos, Vicious Watcher — ruling 2

Gargos's cost-reduction ability affects only generic mana in a Hydra's cost. For example, if you cast a Hydra spell with mana cost {2}{G}{G}{G}, it will cost {G}{G}{G} to cast, not {G}.

Gargos, Vicious Watcher — ruling 3

If a spell has {X} in its mana cost, you choose the value for X before applying cost reductions. For example, if you cast a Hydra spell with mana cost {X}{G}{G} and choose for X to be 6, you pay {2}{G}{G}. Any of that Hydra's abilities that refer to X still uses 6 as the value for X regardless of what you actually paid.

Gargos, Vicious Watcher — ruling 4

Gargos's cost-reduction ability applies only while it's on the battlefield. Gargos doesn't reduce its own cost.

Gargos, Vicious Watcher — ruling 5

Gargos's controller chooses which creature (if any) Gargos will fight, not the controller of the spell.

Gargos, Vicious Watcher — ruling 6

If a spell targets one creature you control multiple times, Gargos's ability triggers once. If that spell targets multiple creatures you control, Gargos's ability triggers once for each of those creatures.

Gargos, Vicious Watcher — ruling 7

Gargos's triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

Gargos, Vicious Watcher — ruling 8

If the target creature is an illegal target when Gargos's last ability tries to resolve, the ability doesn't resolve. If Gargos is no longer on the battlefield, the target creature won't deal or be dealt damage.

Gargos, Vicious Watcher : règles, fonctionnement et interactions (MTG)

Gargos, Vicious Watcher 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

Vigilance Hydra spells you cast cost less to cast. Whenever a creature you control becomes the target of a spell, Gargos fights up to one target creature you don't control.

How it works : Gargos, Vicious Watcher

Original English text — Wizards of the Coast / Scryfall

#1July 12, 2019· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Gargos). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
No other card is named in this ruling.
#2July 12, 2019· Wizards of the Coast
Gargos's cost-reduction ability affects only generic mana in a Hydra's cost. For example, if you cast a Hydra spell with mana cost , it will cost to cast, not .
No other card is named in this ruling.
#3July 12, 2019· Wizards of the Coast
If a spell has in its mana cost, you choose the value for X before applying cost reductions. For example, if you cast a Hydra spell with mana cost and choose for X to be 6, you pay . Any of that Hydra's abilities that refer to X still uses 6 as the value for X regardless of what you actually paid.
No other card is named in this ruling.
#4July 12, 2019· Wizards of the Coast
Gargos's cost-reduction ability applies only while it's on the battlefield. Gargos doesn't reduce its own cost.
No other card is named in this ruling.
#5July 12, 2019· Wizards of the Coast
Gargos's controller chooses which creature (if any) Gargos will fight, not the controller of the spell.
No other card is named in this ruling.
#6July 12, 2019· Wizards of the Coast
If a spell targets one creature you control multiple times, Gargos's ability triggers once. If that spell targets multiple creatures you control, Gargos's ability triggers once for each of those creatures.
No other card is named in this ruling.
#7July 12, 2019· Wizards of the Coast
Gargos's triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
No other card is named in this ruling.
#8July 12, 2019· Wizards of the Coast
If the target creature is an illegal target when Gargos's last ability tries to resolve, the ability doesn't resolve. If Gargos is no longer on the battlefield, the target creature won't deal or be dealt damage.
No other card is named in this ruling.