I've been looking and it seems like no one has ever moved from GnuCash to Quicken!
I know that GnuCash can import .qif's but Quicken cannot import GnuCash's XML files and I can't see any way to convert it.
I've asked Quicken for help and they told me that I had to input all data by hand. I have way too much data to input by hand; I track multiple people's finances.
c'mon guys and gals, someone has to have succeeded in doing this somewhere