The GEL is the 'Gene Emitting League'. Their goal is to clone people. GEL is the Aeta's worst enemy.
The GEL is owned by a man named Doctor Jake Deller. Dr. Deller is a selfish and greedy man that only wants GEL for himself. He killed every scientist on the team, so he could clone himself thousands of times to make employees of himself.
So the GEL is basically owned by Dellers, and the Dellers want to rule the world, so the Aeta hates GEL for that reason.
Before the GEL was taken control of by Deller, they captured Ethan and his family.