On game days, Jimmy Garoppolo, the San Francisco 49ers starting quarterback, usually bounds off the team bus in a svelte suit, looking more like a media mogul than a Super Bowl winner.
His pinched jacket and tapered trousers could well be from some Italian tailoring institution like Zegna.
But their actual origin is often Gentleman’s Playbook, a custom-suit brand run out of a Hoboken, N.J., apartment by Tom Marchitelli, 41, a former hedge-fund accountant.
The brand is barely known to the broader fashion world.
But through locker-room word-of-mouth and furious social-media outreach, Mr. Marchitelli has become a go-to for professional athletes.