Gregor, formerly designated CC-5576 (short for CC-5576-39), served as a clone trooper captain of Foxtrot Group, an elite clone commando squad of the Grand Army of the Republic, fighting the Clone Wars. During the Battle of Sarrish, he was forced to retreat into space. His transport ended up crash landing on Abafar. The crash caused him to suffer from amnesia, and he was rescued by a Sullustan named Borkus who took advantage of his condition to put him to work as a dishwasher in his business. Gregor regained his memories after Colonel Meebur Gascon and his unit identified him after their chance arrival in Pons Ora. He was thought to have been in an explosion trying to protect the colonel and his companions in a firefight against Separatist droids, but survived, although he suffered from brain damage, causing him to partially go insane.

After the Clone Wars ended, Gregor was sent by the new Galactic Empire to a secret facility on the planet Daro, where he and other clone commandos were tasked with training the «TK troopers» which were to replace the clones. Gregor quickly realized he wanted out and sent a distress signal to the clone deserter Rex, who was unable to come himself but sent the Bad Batch, another group of deserters, to rescue him.

Fifteen years after the end of the Clone Wars, Gregor was still alive and was living in a modified AT-TE on Seelos with Captain Rex and Commander Wolffe. In 0 BBY, Gregor participated in the Liberation of Lothal, where the Lothal resistance group and the Spectres drove the Galactic Empire off Lothal once and for all. During the conflict, Gregor was killed, passing away after telling Rex that it was an honor to serve with him and to fight for a cause the clones actually chose to believe in.