Gig to hire doesn't work if the job market is hot enough that potential employees have full-time job offers in addition to your gig-to-hire offer. I've been able to gig-to-hire for offshore developers, but never for onshore.

Got my most recent two employees this way, one sales, one dev, both had prior employment, I'm in Seattle

What's the difference between "gig to hire" and "6 month contract" then hire if you like them?

I don't know about Silicon Valley but in the UK with contractor rates being what they are, the employee will likely make a hell of a lot more money while on that 6 month contract then they will when they get hired permanently. So it may seem to the employee like they are taking a giant pay cut in order to have a perm position.