New Delhi, Oct. 27 -- For investors lucky enough to land shares in Jio Platforms Ltd.'s $3.1 billion IPO, trading on the unofficial market is fuelling hopes of a big debut pop.

Billionaire Mukesh Ambani's digital and telecommunications giant has attracted stellar demand across investor categories ahead of its 28 October market debut. Investors who got their allocations on Tuesday are now closely watching Jio's gray-market pricing for clues about how the first day might unfold. The premium over the IPO price has hovered at about 22% for roughly 10 days, unusually resilient for an offering of this size, according to the ipowatch.in and investorgain.com platforms.

GMPs are unregulated and provide no guarantee of where a stock will actually...