While formatting the email, you may need various formatting options. Gmail offers almost all the formatting option, but it is missing a rarely used option strikethrough. Here is the simple trick to get it to your Gmail.
First Open M.S. Word and Enter the text you want strikethrough in it. Select the text and click on the button shown in the image.
Now copy (CTRL + C) the text and Paste (CTRL + V) in your mail body. See below for the image.
You can also add the extension of Strikethrough in your chrome or any other browser. Else you can use outlook or other similar programs to get various formatting functions.