If it came back in a reply, then either the other person forged the quoted header (unlikely) or it was in the header of your outgoing message.
The header line:
From: Gib Henry gibhenry@gmail.com On Behalf Of gib@gibhenry.com
Means that the message was sent through the GMail SMTP server (probably logged in to your GMail account), with a From: header with your gibhenry.com address.
The GMail server saw that the sending account didn’t match the message’s header and created that “on behalf of” content before transmitting it.
Since this wan’t intentional, I’d review your mail client’s configuration. It sounds like your message was sent via GMail, even though you selected your domain as the message source.
There’s another thread here discussing a similar problem with Apple Mail, where it’s sending messages via the wrong SMTP server. So far, it looks like you’re seeing another symptom of that problem.