Strikethrough (or strikeout) texts means that the words are wrong. They are often represented by words with a horizontal line across their center. Something like this. However, most social media platform like Facebook, Twitter and even Gmail (a few months back) don't offer any native way to add strikethrough text. Thankfully, there are few simple ways to add strikethrough text in Gmail. Here's how. Option 1: Use the native editor As of 2019, Gmail has started offering Strikethrough text natively. Though it's hidden a bit under the settings. To use this option, first, select the text to strikethrough in the Gmail compose window. Next, click on the more formatting option at the bottom (the bold A with an underline), a new line of formatting options will pop-up. On the right-most part, you'll see a drop-down arrow, click on it, and the second option from the top should be strikethrough. strikethrough Since this feature is now built-in, you don't have to rely on external sites or browser, as it should work for all browser (tested on Chrome and Safari on Mac). But unfortunately, it didn't work on the mobile version of Gmail. Option 2: Use the keyboard shortcuts. Even though there is a native option, I prefer to use keyboard shortcuts to toggle the strike-through. It's much faster that way. Although, it only works on Chrome browser. Select the text, and use the following keyboard shortcut to toggle the strike-through. 
Windows: Alt+Shift+5
Mac: Cmd+Shift+x
Option 3: Use an external editor While using an external editor is not as faster as a keyboard shortcut, it gives you many markdown editors that Gmail lacks. Simply copy-paste your email to CKeditor demo and format it the way you want. Once don, copy the final text into Gmail - the formatting remains intact. Also, since it's a web app, it works on a mobile browser as well. CKeditor Also read: How to Print Email From Gmail Without Header