Gruul Beastmaster

• WWizards of the Coast2019-01-25
  Official Ruling
  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.
• WWizards of the Coast2019-01-25
  Official Ruling
  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.
• WWizards of the Coast2019-01-25
  Official Ruling
  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.
• WWizards of the Coast2019-01-25
  Official Ruling
  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.
• WWizards of the Coast2019-01-25
  Official Ruling
  If a creature entering the battlefield has riot but can’t have a +1/+1 counter put onto it, it gains haste.
• WWizards of the Coast2019-01-25
  Official Ruling
  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.

Rulings via Scryfall