What is the surname of King George VI?

Monarchy Monarchs do not have a "last name" as we understand it nowadays. King George I of England belonged to the Royal House of Hanover.

Surname: Guelph

The Lives of the Kings and Queens of England,

Page 262, Paragraph 3, ISBN: 0297 769111

Also This time line seems to suggest the same: google.com/search?q=king+george+guelph&hl=en&client=firefox-a&rls=org.mozilla:en-US:official&hs=UZe&tbs=tl:1&tbo=1&ei=6AdVSuXcIZ7cswOJruCNCQ&sa=X&oi=timeline_result&ct=title&resnum=11">http://www.google.com/search?q=king+george+guelph&hl=en&client=firefox-a&rls=org.mozilla:en-US:official&hs=UZe&tbs=tl:1&tbo=1&ei=6AdVSuXcIZ7cswOJruCNCQ&sa=X&oi=timeline_result&ct=title&resnum=11