Google Docs has more editing features than the Gmail compose window. Moreover, it is easier to have several revisions and reviews with Google Docs. So if it is an important email or an email that requires some extra features (such as tables, multiple embedded images etc) it is a good idea to compose the email in Google Docs.
However, how will you get the Google Docs contents inside the email?
Embedding your Google Docs contents in the email
The technique is simple. Open your Google Docs and compose the email. You can use all the editing features that Google Docs has.
Now do Select All (menu item: Edit → Select All)
and do Copy
Paste the contents to Gmail Compose window.
It will retain all the formatting, including constructs like the table:
Then send the email!