gmail is an smtp email server so the smtp option makes sense. FileMaker users have had mixed success with gmail as an smtp server. Some have had no trouble, others lots of trouble.
To preview and edit the message body before sending, have your script put the text and data you want to include in the body into a text field. You can then review and edit this text field before clicking a "send" button that uses Send Mail to send the email. The send mail step can be set up to refer to this text field in order to send it as the body of your email. (This also gives you the option of saving these messages in a table for future reference.
To copy data to the system clipboard, there's a script step named "copy" to do exactly that. The field being copied must be present on the current layout in order for it to copy data. This is useful if you want to paste the copied data into another application. If you want to copy data from one part of a FileMaker solution to another, there are better options than copy for doing this.