The preferred storage data type for GUID values on Oracle is RAW(16).
A GUID is not a number in the sense that your height is a number -- you are not going to add them together, or take their average, for example.
http://docs.oracle.com/cd/B28359_01/server.111/b28286/functions175.htm
http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:77564387253035