Gruul Beastmaster — ruling 1

The value of X is determined only as Gruul Beastmaster’s triggered ability resolves. Once that happens, the value of X won’t change later in the turn even if Gruul Beastmaster’s power changes.

Gruul Beastmaster — ruling 2

If Gruul Beastmaster leaves the battlefield before its triggered ability resolves, use its power as it last existed on the battlefield to determine the value of X.

Gruul Beastmaster — ruling 3

If Gruul Beastmaster’s power is negative as its triggered ability resolves, X is considered to be 0. In other words, that ability can’t cause the creature to lose power.

Gruul Beastmaster — ruling 4

Riot is a replacement effect. Players can’t respond to your choice of +1/+1 counter or haste, and they can’t take actions while the creature is on the battlefield without one or the other.

Gruul Beastmaster — ruling 5

If a creature entering the battlefield has riot but can’t have a +1/+1 counter put onto it, it gains haste.

Gruul Beastmaster — ruling 6

If you choose for the creature to gain haste, it gains haste indefinitely. It won’t lose it as the turn ends or as another player gains control of it.

Gruul Beastmaster : règles, fonctionnement et interactions (MTG)

Gruul Beastmaster 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

Riot (This creature enters with your choice of a +1/+1 counter or haste.) Whenever this creature attacks, another target creature you control gets +X/+0 until end of turn, where X is this creature's power.

How it works : Gruul Beastmaster

Original English text — Wizards of the Coast / Scryfall

#1January 25, 2019· Wizards of the Coast
The value of X is determined only as Gruul Beastmaster’s triggered ability resolves. Once that happens, the value of X won’t change later in the turn even if Gruul Beastmaster’s power changes.
No other card is named in this ruling.
#2January 25, 2019· Wizards of the Coast
If Gruul Beastmaster leaves the battlefield before its triggered ability resolves, use its power as it last existed on the battlefield to determine the value of X.
No other card is named in this ruling.
#3January 25, 2019· Wizards of the Coast
If Gruul Beastmaster’s power is negative as its triggered ability resolves, X is considered to be 0. In other words, that ability can’t cause the creature to lose power.
No other card is named in this ruling.
#4January 25, 2019· Wizards of the Coast
Riot is a replacement effect. Players can’t respond to your choice of +1/+1 counter or haste, and they can’t take actions while the creature is on the battlefield without one or the other.
No other card is named in this ruling.
#5January 25, 2019· Wizards of the Coast
If a creature entering the battlefield has riot but can’t have a +1/+1 counter put onto it, it gains haste.
No other card is named in this ruling.
#6January 25, 2019· Wizards of the Coast
If you choose for the creature to gain haste, it gains haste indefinitely. It won’t lose it as the turn ends or as another player gains control of it.
No other card is named in this ruling.