"Gross margin" is not the same as "hold" here.

Options pricing is reasonably competitive. Even a gambly thing like a Tesla zero day option has a spread of 1-2%, so someone trading it at random loses 0.5-1% per trade. And Robinhood is a brokerage, not an options market maker, so it doesn't capture all of that 0.5-1%.

You'd have to read Robinhood's financials to see what they mean by gross margin. Possibly it means if a customer deposits $1,000 and trades options, the customer eventually on average loses $900 of it? Even that seems too much TBH.