1) Go to full site for Gmail on a computer and click on the gear in the upper right. Go to settings and select 100 conversations per page (this is the max).

2) type in the following in the Gmail search field “is:unread” (without the quotes).

3) Click on More and “Mark all as read”

4) Another 100 should appear and repeat step 3, repeat until there are no more messages in the “is:unread” search. I did this for 3000 and it didn’t take too long.

Thanks – Joe