Most email clients set limits to the size of emails' HTML code. For Gmail, it is 102Kb.
I.e. when your email size exceeds this volume, Gmail automatically "clips" your email. To avoid this, please reduce your email weight by removing some content in containers and structures, and also by minimizing the number of styles applied to email elements. For instance, the "Support for Outlook" option alone is 1KB per button in the email. Disable this option to prevent your emails from being clipped in Gmail.
And export your email as HTML file...
Toggle the "Minimize HTML" button, this will reduce the weight of the email template file. Please note, all line breaks, spaces, and extra characters will be removed, but this won't affect the way your email content displays.
Then open this file with any text editor and copy/paste the HTML code into the service you need for further sending to Gmail.
Please be advised that some elements of your compressed emails may look incorrectly across some email clients. So please remember to test your emails before sending over to recipients!