In Episode 80 of the 2011 anime, they showed Gyro's back story then a clawed up chair. Was he killed or was he eaten? It wasn't clear what happened to him.
Gyro is killed and eaten after the Chimera Ants invade NGL.
but he is later reborn as a Chimera Ant... and the first to keep all of his human memories and break free of the Chimera Ant Queen's control.
The Chimera Ant King, Mereum, cannot be Gyro, because Gyro was shown leaving the hive at the same time that Gon was training to defeat Knuckle, in Chapter 204, before Mereum was born.
According to the manga, he became a chimera ant and had all his human memories intact. He escaped while the queen was dying and supposedly went to Meteor City to start his empire again (according to the manga). I think they might have him return in the future as a super villain or something, because in ep 79 or 80 it showed his back story and his thoughts on humans. He wanted to bring evil in his human form, so why not in his chimera ant form?
Gyro was not re-born as the Chimera Ant King, Meruem. Gyro was killed and eaten by the Queen, turned into a Chimera. But he re-gained every single one of his memories and fled to Meteor City to rebuild his Empire. It says so in the manga and on Gyro's Wiki page.