is it possible to duplicate a site?

No 😞

You can duplicate pages within a site, but not an entire site

I recommend adding your upvote and use case to our product board, if you have time! https://meetgamma.canny.io/ideas/p/let-me-duplicate-or-delete-my-gamma-sites

Not all at once. You can do it card by card. That's probably the easiest way today