Ugh. Lacking an edit button....sorry for double post.
Anyways, in the Voldemort Conflict, you can make an assumption that the majority of his inner circle were Slytherins, based off the fact that it seems they all had kids around Harry's age. What a remarkable coincidence...
Now, what conclusion can you draw from that? Only that Riddle, being a Slytherin, recruited from the people he spent time with, other Slytherins. If Riddle was a Gryffindor, you could bet that most his inner circle would have been too.