The plot twist: The one who changed your gmail password is the seller of those accounts.
I'm bit confused why you had to buy gmail account if you could make it yourself. Well mate, I think there is no solution for that.
Thats why you should check everything after you buy an account that you are the sole owner and previous owner has no scpoe of recovering it back.
Also all 100 gmail account changed password?
all you need do is that click on that particular account,click on forgetting password of which you be ask some question like your recovering email like alternative email so they can send you the password.
And who will hold that alternative email in this case?