Galen did in fact had a good deal of stamina and endurance. One had to have it after all in being Vader's weapon and tool. Though if asked he was fairly certain Gaia had equal stamina and endurance to match in this pleasurable activity. When she started moving faster and harder, he matched her. Still having barely enough control to thing to experiment, he nipped at Gaia's neck, wondering how she would react to that.
If he had been of clearer mind earlier and not been so panicked about Rikki, Galen might have foreseen this in his meditation. But worry and sense of guilt had led to him skipping over meditating. That he minded all that much. This was definitely a good distraction to have and he could feel the pleasure swelling more and more in him. Somewhere in the back of his mind he knew that meant he was close to orgasm. But he didn't have enough control to inform Gaia of that.
Instead he gripped her tighter and began moving harder with his hips, as if he were trying to get deeper in her.