I love Gregory. It's in my top 5 and climbing. I would call Greg too outdated, but Gregory in full, or with a different nn like Gregor or Rory, not so much. That said, I love "mid-century" names (I could easily name a dozen children off the top 100 list of 1950), so it's possible my perception is way off base.
I like Cosmo, but I'm not crazy about some of its associations: Kramer from Seinfeld, the very pink drink, the magazine. I prefer Cosmas. Both Cosmo and Cosmas would be unusual, slightly quirky choices, but I don't think either one is too out there.
I've known a handful of Gregs/Gregorys and one elderly Cosmo.