A lot of email clients set message size limitations. With Gmail, the limit is 102Kb.
I.e. when your email size exceeds this volume, Gmail automatically "cut" it. To avoid this issue, please reduce your email weight.
You can also export email in HTML format...
... using code compression.
Then open it with any text editor and copy/paste the HTML code into the service you need for further sending to Gmail
Please note that when you use the compressed code, some email's elements may not be displayed correctly in some email services. So please do not forget to test your emails before mailings!