Consider: if Gaster had been alive during a genocide run he probably would have lured Frisk into the true lab and trapped them in the determination extractor
That would actually prevent frisk from being able to reset by draining their determination. Also, he could probably synthasize a safe varient of determination and bring back those frisk killed without turning them into amalgamates. Also, they would be able to break the barrier and finish the war with nigh immortal hypergods of death that only get stronger with each human they kill.