I work for a small non-profit and we've had the same issue. We use SendGrid to send out our email newsletters and the open rates have fallen from ~27% to ~13% over the past month.

Does anyone here have contacts at Google who might be able to help? [Edit: I've also reached out to SendGrid support to look into this]

Looking at the data more, the drop is due to a ~73% drop in Gmail opens. Hotmail and Yahoo opens remain stable:

1) See drop in overall open rate here: https://www.dropbox.com/s/j8grg3r684zshhz/Drop%20in%20open%2...

2) See drop in Gmail open rate vs. Yahoo and Hotmail here: https://www.dropbox.com/s/0fdvbddwno27mgf/Drop%20in%20Gmail%...