The Gmail integration includes a NetSuite bundle side component with scripts that are used to attach the email. NetSuite jobs are allocated a certain number of 'points' and must finish before they are used up.
In certain cases with a large number of images, an extremely large message body with multiple threads, etc., the NetSuite script runs out of points processing the email.
The following workarounds may help:
Forward a copy of the email to yourself and reattempt attaching it to NetSuite
Disable inline image sync
Available via Setup>Company>General Preferences>Custom Preferences as shown below.