Q:

Gmail not showing correct font

I am trying to change the font of my emails to open sans, however, I am having issues with Gmail rendering the correct font.  I managed to find a way to solve the issue for outlook.  This is what I have:
body {
        @font-face {
                font-family: 'Open Sans';
                font-style: normal;
                font-weight: 300;
                src: local('Open Sans'), local('OpenSans'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3bO3LdcAZYWl9Si6vvxL-qU.woff) format('woff');
            }
        }

I have also tried other things such as declaring all text to be open sans:
<link href='http://fonts.googleapis.com/css?family=Open+Sans:300'rel='stylesheet'type='text/css'>
     <style type="text/css">
    .stylealltext { font-family: 'Open Sans', sans-serif;}
    .stylealltextbold {font-weight: bold; font-family: 'Open Sans', sans-serif; }

However, Gmail decides to show Arial, while Google Inbox decides to show Helvetica.. It looks like Google's mailing services just overrides whatever I feed it.  Can anyone help? 

Solution
Salvimatus's solution worked:
<span style="font-family: your-chosen-font">your text</span>

The span needs to manually added. 

A:

Gmail does not support @font-face. 
For you customize the font, you should write inline in each block of text that you have:
< span style="font-family: your-chosen-font">your text< /span>
Note: must be a font we have natively on Windows or Mac. 

A:

Styling in-line is definitely best practice however it would seem the major email clients do support adding a font declaration to CSS included in the head of the html file.  
As Aaron mentions custom fonts do not work in emails and most likely never will for the foreseeable future.  Avoid this by discussing beforehand with clients or designers.  Only websafe fonts will display in email clients and even then you will get mixed results in leading and kerning cross platform. 
Salvimateus is correct that the following will be a bulletproof solution.  
< span style="font-family: your-chosen-font">your text< /span>

However this will also work:
<table>
  <tr>
    <td style="font-family: your-chosen-font;">Your text here <br> more text.</td>
  </tr>
</table>

As will this:
// CSS added to the <head> of the email
<style>
  .myClass{ font-family: your-chosen-font; }
</style>

// Class added to the <td> element in the <body> of the email
<table>
  <tr>
    <td class='myClass'>text here <br> more text.</td>
  </tr>
</table>

I can confirm the last option seems to work in the major email clients (Gmail, Outlook, iOS) both desktop and mobile and I've seen good results on Windows phones also.  
This is mainly due to the email being recompiled by the 3rd party email marketing software you are naturally using and the css is shoved inline by the compiler.  
The net result here is less initial work for the developer.  
As a side note if you happened to add all three (just to be sure) - then Salvimateus solution will trump all others as it is applied to the <span>.  In this instance it is read last and therefore rendered by the email client.