Page 9 of the GMG (in the blue box (Higher Attributes) tells me that traits above 6 cost the same (but require a gateway trait. Page 111 however (also in the blue box) says Attributes above 6 cost two character points each.
I've not seen any official clarification of this. The way I work it is that with a gateway trait, Attributes above 6 may be bought at the same cost. Without a gateway trait, Attributes above 6 cost 2 points each, with the restriction that this is not available to starting characters (i.e. above 6 can only be purchased by using experience points awarded by the GM).