South Korea and Japan are specifically different because they never had GSM networks; elsewhere in the world, I'm not specifically aware of any GSM networks already being shut down (so, suggesting a GSM phone for Europe won't be wrong at all), although I do recall that several operators that have started out since 3G UMTS has already been available, are likewise offering a UMTS-only service with no GSM (»three.co.uk
Also, some operators switched to UMTS from some other crappy 3G (»bell.ca
, they have a shared UMTS network, IIRC); Verizon delayed the switch, but went directly with LTE with no UMTS. When I was in Canada in 2008/2010, I had an unlocked GSM phone from AT&T, Sony Ericsson W810i, and it wouldn't work with Bell or Telus, but would only work with Fido and Rogers.
And FYI: SIM cards don't mean GSM, either. There are some SIM cards that won't work in any GSM, UMTS or LTE phone, so, even without UMTS: for sure SIM and GSM aren't interchangeable terms at all, I'm again not sure what made you to believe they were.
Let me put it this way: I need a replacement phone for Verizon; you come by and suggest that I can just get a GSM phone, since Verizon has nationwide LTE, and LTE is a GSM descendant -- please keep such advice to yourself, cause it's wrong.
South Korea has no GSM, period.