gb2 is short for go back to. It is used in the format of gb2/*/ where "*" is the section of 4chan or other imageboard. ie: gb2/b/ for /b/tards. Not limited to 4chan/imageboard usage, e.g. "gb2/ytmnd/", "gb2/suicide/".
/*/ is that way
/*/ is that way is a more polite way of saying gb2/*/.