ScareMail is an extension to add scary sounding text to the end of emails to waste the NSAs time sifting through garbage.

https://bengrosser.com/projects/scaremail/

It looks like it's really just trying to make a point instead of attempting to actually be effective, as it puts the text in the signature section, which could easily be filtered out. Plus it identifies itself as ScareMail, which makes it even easier to spot and exclude.

I don't use any web mail, let alone google's webmail, but if you are more interested in making a point to the people you email (to invite them to ask why that text is there), maybe you will find it interesting.