Barcodes are broken in Gmail

Are you saying Gmail removes CSS classes from the markup? That doesn't seem like it has anything to do with this module. That seems like a problem in the mailer software you're using in Drupal. Or are you saying the css files are not included and inlined into the email? That again would be a problem with your Drupal mailer module.

Are you aware that Drupal can't send HTML emails without a helper module?

Please provide the complete raw markup of one of these emails as sent, including headers.

I concur that the issue is with the mailer module (symfony). I have posted an issue there. My apologies and this issue can be closed.