I used gmail on a semi-dedicated machine that is listed in the headers of emails sent out

Received: from someserver.semidedicatedhost.com

instead of

Received: from mymaildomain.com

This causes other mail host that are receiving my mail to think of it as spam or bulk. mymaildomain does not resolve someserver.semidiedicatedhost.com

Where in gmail can i edit the header information of the emails going out?

thanks

Those headers aren't added by Gossamer Mail, but your mail server. You need to reconfigure your mail server.

Adrian