There may some emails related issues occur while forwarding Yahoo mail to Gmail account then just follow some simple steps:
1. Login Gmail
2. Now, look at the section that allows marking emails or provides actions.
3. Now choose the filter message and select the box that never marks those emails as spam.
More help visit: Yahoo phone number
Yahoo Toll-Free Number
+888-815-6317(US/CA)
+44-800-051-3717 (UK)
+61-180-082-5192 (AUS)