Is the Apple SD Gothic font free to use on the web?
The bad news seems to be: probably not.
If you treat one of the OTF fonts in the Apple SD Gothic family as plain text and try to open it in a text reader such as TextEdit, inside the code of the font itself one may read this informative text:
“Copyright © 2011 Sandoll Communications Inc. All rights reserved.
Apple SD Gothic Neo is a registered trademark of Sandoll Communications Inc. (산돌은 폰트를 짓습니다, 산돌커뮤니케이션)”
Nowhere does it specifically say that this font is permitted to be used as a Web font.
Evidently, Sandoll has created this font family and, presumably for a fee, has licensed the fonts to Apple to be used on Apple hardware. Apple lists the font family as being installed with OS X and iOS (OS X: Fonts included with Mavericks, iOS 7: Font list).
Microsoft Windows and their MS Office product may install Korean-capable fonts, but their Language Packs page does not list this specific family. Presumably, Microsoft could include the font as well if they wanted to, but they would need to license it from Sandoll first. (Page on microsoft.com)
Wikipedia, which describes Apple SD Gothic Neo (애플 SD 산돌고딕 Neo) as the “default Korean font on Apple Mac OS X 10.8 Mountain Lion and iOS 5.1,” does not list it as free and open-source. (List of CJK fonts)
Other sans-serif Unicode fonts suitable for Korean, however, are available which are free and open source, including UnDotum (은돋움), UnShinmun (은신문), Baekmuk Gulim (백묵굴림), Nanum Gothic (나눔 고딕), Hamchorom Dotum (함초롬돋움) and Jieubsida Dodum (지읍시다돋움). Many of these non-proprietary fonts are derived from Linux projects.
