I tried inserting a MS Excel document directly / via a Microsoft document but when copied into a new Gmail message the formatting wasn't retained.
How do you insert an Excel sheet as part of a new mail message?
As Gmail doesn't preserve formatting, your options may be to:
Does it have to be an editable sheet? If not Excel can save as PDF or as a webpage to maintain the formatting
Copy what you want from the Excel document to MS Word first, and then try to paste it into Gmail by coping it from MS Word.