1. Make bad decision
2. Revert all back as a new feature

Great news (at least for me). With the latest LT update, it works perfectly with V7 Gmail Zoom!

FYI

I have tried to use

transform: scale(1.2);
transform-origin: left top;

instead of

zoom: 1.2;

A little red circle is still dislocated, but a dropdown menu on incorrect words is OK now.

Great addon, but it breaks the work of the LanguageTool addon in the Compose window.