Gmail made some changes to their system, and now images are displayed automatically when an email is opened. This is good news for you- you'll probably see a better open rate for your Gmail readers than you have in the past.
VerticalResponse counts an email as "opened" if the images are displayed in the email. That means that in the "old" Gmail, if a recipient opens the email and reads the text without choosing to display the images, it will not be counted as "opened" in your VR reporting. With the "new" Gmail your open rates will be more accurate because images will be displayed automatically when your recipient opens an email in Gmail.
For more information, please check out our blog about Gmail's new automatic image display here.