The codename was spotted by leaker Moore's Law is Dead and reported by IT Home. It carries extra weight because the original Steam Deck's chip was codenamed Aerith, after the Final Fantasy VII character, and Gainsborough is Aerith's last name in the game.
The Verge's Sean Hollister, however, writes that he is "a tad skeptical" the leak means a sequel is imminent. A driver entry confirms only that AMD has tested a chip with that name; it does not confirm the chip has reached production, that it delivers the leap Valve is asking for, or that a device built around it is ready. Still, the discovery is the strongest sign yet that a successor chip is in the works. Handheld gamers waiting for a more powerful Steam Deck will be watching for official confirmation from either company.