I was struggling with the same issue. The only solution I found was adding a script that built the html for the grid for me and then adding the html to the content. I think it was a bit of an overkill though and would not do it again unless strictly necessary (you have to resemble exactly the gmail HTML format). On the other hand, adding small HTML changes in the email content is much easier (you can embed links, add embedded images, padding, etc. in the email content) by just adding basic HTML in the email content.
Hope this helps!
Calendly: Calendly - Alessio Monino