Hi,
Hopefully your "email" is of type EmailMessage. If that is the case you should be able to do the following:
EmailMessage email = new EmailMessage();
email.From = "SomeoneOne@gmail.com";
EmailSender.SendEmail(email)
Additionally, you can get an email template that you have defined in Kentico through the portal and use that to build the subject/from/to/body of your email message. If you have a "from" address defined in the template you can add that "from" address to the Email Message like this:
EmailTemplateInfo template = EmailTemplateProvider.GetEmailTemplate("Template Name", "CurrentSiteName");
EmailMessage email = new EmailMessage();
email.From = EmailHelper.GetSender(template, "defaultsender@gmail.com");
EmailSender.SendEmail(email)