Gmail – Paste a webpage in Gmail body

Gmail doesn't support it out of the box. The only possibility is using some software like Thunderbird and send the mail via SMTP to your mail-account.

I can confirm that Gmail does allow you to send tables, but provide very limited editing support.

What I would do is to copy the whole e-mail into an online HTML editor like TinyMCE and do the editing there. You can also copy it a local tool that supports HTML editing, like Word or Dreamweaver.