It's been a month since you posted Gizmo, so you've probably already figured out how to do this, but if you haven't yet, or for the benfit of others:
If you want to populate the From field with an email retrieved from the form, you can just add the following to \includes\processing_functions.php >
ninja_mail_form function in the first
$email_from = str_replace("[$label]", $val, $email_from);
It needs to be underneath the following lines:
$id = $post['id'];
$label = $post['label'];
$type = $post['type'];
$val = $post['value'];
$extra = $post['extra'];
Then in the admin area you can just set the Email Sent From field to [Name]<[Email]>. (You'll need to replace [Name] and [Email] with whatever names you gave to these fields on your form).
Would be great if it could be implemented in a future version as I really don't like having to modify plugins this way.