https://tlc.thinkorswim.com/center/charting/studies/studies-library/M-N/MajorGannLevels.html
http://www.investopedia.com/articles/trading/08/gann-indicator.asp
 

EDIT:
This posting is nice: http://www.traderslaboratory.com/forums/technical-analysis/9711-discussion-gann-2.html#post119246
... as it brings in the factor of time. Many traders seem to forget that (only thinking of: "Get rich quick!" - but what is "quick" anyway?  ) - especially if you look at what is discussed in the chat-box on Cryptsy sometimes, for example.