If the extension is valid, it'll be loaded up and active right away! If it's invalid, an error message will be displayed at the top of the page. GMass is one of the most preferred Gmail extensions used by companies to send cold emails.
![the best gmail addons the best gmail addons](http://i1.ytimg.com/vi/feNbGXiHgdg/maxresdefault.jpg)
![the best gmail addons the best gmail addons](https://blogcdn.gmass.co/blog/wp-content/uploads/2020/11/Gmail-Add-Ons-Gmass-Personalization.png)
and select Extensions under the Tools menu to get to the same place).