Complex sentence: Gibbs invited the pro-slavery president of Dartmouth College, Nathan Lord, to give the ordination sermon, begged Dr. Lord as a special favor to preach his ordination sermon, giving as a reason that "his" college was the only which would endure his presence.
Split into simpler sentences: Gibbs invited the pro-slavery president of Dartmouth College, Nathan Lord, to give the ordination sermon. He begged Dr. Lord as a special favor to preach his ordination sermon, giving as a reason that "his" college was the only (one) which would endure his presence.