I found nothing restricting gold certification to those who previously had Silver so I went ahead and just got gold.
As far as whether you're missing out on anything important by skipping out on silver, I can't tell you from experience but from what I've read here, the focus is more on shopper ethics than useful shopping education.
And even though you didn't ask whether gold certification is worth it, I'm not entirely convinced one way or the other. I'm in the midst of a 6 month trial documenting my experiences before and after certification and I can't say it has yet completely proven to be worthwhile for me. But I believe it can be and probably depends a lot on location and other circumstances.