Hi Pablo,
Thanks for your reply!
It looks like the unsubscribe link is showing up in gmail after all (just because I have it in the footer of the email):

Here is my code for the opt out link at the bottom of the email:
<% if( document.mode != 'forward' ) { %>
<table cellspacing="0" cellpadding="0" border="0" align="center" style="width:100%">
<tbody>
<tr>
<td style="font: bold 11px Corbel, Verdana, Frutiger, Helvetica,sans-serif; color: #ffffff; text-align: center; background-color: #00703c" valign="top" >
<p style="font: normal 11px Corbel, Verdana, Frutiger, Helvetica,sans-serif;">You've received this email because you are part of <br><%@ value object='branding' xpath='@publicLabel' %>’s email list.<br> If you wish to unsubscribe, please
<a href="http://support.nature.org/site/Survey?ACTION_REQUIRED=URI_ACTION_USER_REQUESTS&SURVEY_ID=17753" _label="Opt-out link" _type="optout" style="color:#ffffff;">click here</a>.
</p>
</td>
</tr>
</tbody>
</table>
<%}%>
Would having a second opt out link at the top as well help?
I figure also I still need a "mailto" unsubscribe link. For example, this litmus article shows the following in an email header:

But I don't quite understand how a mailto unsubscribe works.
Thanks!
Danny