Now, before you cry, "But I have 10,143 unread messages. Please don't tell me that I have to mark messages in groups of 50!" allow me one further tip.
In your Mac's browser navigate back to your Gmail page and enter is:unread in the mail search field. Just below this field is the Select box (the one with the square and downward-pointing arrow). Click it and choose All. A string of text will appear above your first message that reads 'Select all x conversations in Inbox' (where x is the number of unread messages). Click this link, and you'll select all unread messages, even those that don't appear on the current page. Now click the More box and choose Mark as Read. After cogitating, Gmail will do exactly that. Now, as I explained earlier, return to Mail on your iPhone, refresh your email, and all your messages will be marked as read.
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