Jo has never directly *said* that James *was infact* in Gryffindor (and on the subject, minor spoiler, um, James a Seeker anyone?)

[Kirstini]

No: HPFGUIDX 65864

 - Yeah, but we got canonical confirmation in OoP:
(Can't find the page at the moment cos the book's too big - someone 
can back me up, please?) "Harry was forcibly reminded of another 
Grffindor Quidditch player who had sat under that very tree, 
ruffling his hair" - the first GQ player being Ron, the "other" 
James.
Kirstini