I had an unclear situation in one of my sessions. A monster grappled a PC adjacent to him. In the next turn, a PC cast Gust on the monster. After failing its strength check the monster was moved away from its prey, outside its 5ft reach range from the PC it had grappled. Is it OK to consider this to end the grappled condition?
Yes, this cancels the grapple
The basic rules note that:
If you succeed, you subject the target to the grappled condition. The condition specifies the things that end it, and you can release the target whenever you like (no action required).
And the grappled condition specifies:
The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.
If one of the creatures is moved such that the grappled is no longer within reach of the grappler, that ends the grappled condition. It doesn't actually matter which creature is moved.
If the monster is moved out of reach, as it is the case in your example, then yes that ends the grappled condition.
A grappled creature’s speed becomes 0, and it can’t benefit from any bonus to its speed.
- The condition ends if the grappler is incapacitated (see the condition).
- The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.