Essentially the open tag will be cached at Google's end, so you'll get a less accurate (lower) count of total opens from Gmail.

Hotmail have been doing this for a while, and how it works is that two opens in quick succession will generally only count as one, but if there are a couple of days between opens then it generally does count as two, because they don't hold on to the cached images forever.

Therefore your open rates will only be slightly inferior.

On the other hand, your click through rates won't be affected, since those are tracked via links requested directly from the user's web browser (outside the Gmail environment).