Tested in TG.
When Gruul Warbeast is in the battlefield, upon playing another creature that does not have haste, already, post-arrival, Haste should be added to creature per text.
Note: If the creature ONLY gets Haste if there is an opposing creature, then the text is incorrect.
Currently reads:
Whenever you cast a nontoken creature, that creature gains Haste, then deal damage equal to its power to target opposing creature.
If it only gives haste when an opposing creature exists, text should be switched to:
Whenever you cast a nontoken creature, deal damage equal to its power to target opposing creature then that creature gains Haste.