>>387
/b/ is the name of the "random" board on many English 2ch style sites, where anything can be posted (it is like nijiura.) If someone says /b/ they are probably referring to the /b/ on 4chan.org, generally regarded as being full of idiots and racists. "gb2/b/" is an abbreviation of "go back to /b/", and saying it to someone implies that they are acting stupidly.
ITT is an abbreviation of "In This Thread."