General Atlantic Nears Investment in $1.3 Billion Gymshark Brand

General Atlantic is nearing a deal to acquire a stake in Gymshark Ltd., the U.K. fitness brand set up by a group of high-school friends in a garage less than a decade ago, according to people familiar with the matter.

The private equity firm is in advanced discussions to invest roughly $300 million in Gymshark, according to the people, who asked not to be identified because the information is private. They have been discussing a valuation of more than 1 billion pounds ($1.3 billion) for Gymshark, though exact terms are still being negotiated, the people said.

An agreement could ...