Q: My gmail sends blank messages to the sender when I receive an email.

Are you sure that you have turned it OFF?

(it sounds as if you have it ON, but have not composed any message body "content" - thus the 'blank' email)

Out of office or vacation reply - Computer - Gmail Help

https://support.google.com/mail/answer/25922?co=GENIE.Platform%3DDesktop&hl=en

Gmail Help Forum - Google Product Forums

https://productforums.google.com/forum/#!forum/gmail

2nd link, Wilson... this has nothing at all to do with any apple device or software

With that said, you have done what I would have recommended next = toggle ON > Save > toggle OFF > Save - that IS what you describe having done, right?

Just a thought: does your ISP or your email provider/app support read receipts? It's a stretch, but it could be Gmail's way to show that you've opened the email if the sender requested one.