what does Gatsby tell the group that causes nick to have a complete renewal of faith in him?

Tom is pushing Gatsby about his time spent in Oxford. Tom wants Gatsby to lie but Gatsby is more humble about it than expected,

“It was in nineteen-nineteen, I only stayed five months. That’s why I can’t really call myself an Oxford man.... It was an opportunity they gave to some of the officers after the Armistice.... We could go to any of the universities in England or France.”

Nick is proud of Gatsby for being truthful.