I haven't delved deeply into the stats around gig workers, but what I do know is that group's data isn't tracked that cleanly, so it's quite possible they are a part of the reason the gap in the workforce stats. To my understanding gig workers are counted in BLS stats, but as part of the larger group of workers who are self employed or part time, so it's not parsed out in a way that's useful from an analysis perspective.
In terms of general stats that I've seen, there are about
59m gig workers of which 17.1m have this form of work as their primary work. What's interesting in the link I've provided is that this form of work pays less than being a full time employee on average, so clearly those choosing it aren't making this decision purely on financial criteria.