In the search pan start typing label:unread
After you type label: you will have some options. More on that later.
After you select label:unread it will search your entire account and find ALL unread messages. No more hidden gmail messages!
Select all you want to mark as read.
Tap the more option (triangle) at the bottom and tap "Mark as unread"
Voila!! No more badges on the Gmail icon!
You can also search for specific people using the from: or to: tags. label:read, label:spam, and label:important are some others to help narrow your search!