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.

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.

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.

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.

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

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.