VULNERABILITY DETAILS
XSS vector gets executed in Google Mail, Google Chrome Version 51.0.2704.84 on 12th Jun,2016 CET

VERSION
Chrome Version: 51.0.2704.84 stable
Operating System: Windows 7

REPRODUCTION CASE
1. Login to your Google account using Google Chrome browser (Tested with Version 51.0.2704.84 on 12th Jun,2016 CET)
2. Install Mail Track extension to Google chrome Browser. (https://chrome.google.com/webstore/detail/mailtrack-for-gmail/ndnaehgpjlnokgebbaldlmgkapkpjkkb?hl=en)
3. Now visit https://www.google.com/maps/
4. Click menu and go to Your places -> Maps and click create maps
5. Create map with title as <IMG SRC=x onerror="javascript:alert('Hacked by Suhas Gaikwad ;)');">
6. Share map with Victim or self.
7. Onload of gmail XSS gets executed.

Tested with following vectors
a. <IMG SRC="javascript:alert('XSS');" onerror="javascript:alert(document.cookie);">
b. "><img src=x onerror=alert(1)/>
c. <IMG SRC=x onerror="javascript:alert(document.domain);">
d. <img src=x onerror=window.open('https://www.google.com/');>

Or Simple way , attacker can able to send email to victim with subject as <IMG SRC=x onerror="javascript:alert(document.domain);"> OR <img src=x onerror=window.open('https://www.google.com/');>
and it will get executed. 

Please refer video at : https://youtu.be/cjUWbz0vy1s

Attack senario :
Ask victim to install Google chrome - MailTrack extension and create Google map or send email with XSS vector as a title to victim. e.g. :  <IMG SRC=x onerror="javascript:alert(document.domain);">

Also using xss vector as  <img src=x onerror=window.open('https://www.google.com/');> , attacker can able to do OpenRedirect in Gmail.

This issue is because of MailTrack extension, disabling MailTrack extension mitigate XSS vulnerability. 

Let me know if you need more details. Thank you.