Gpg4win is a gui front end to GPG (Gnu Privacy Guard). It is used to encrypt, decrypt, and digitally sign files.
It’s compatible with PGP. I managed to load my way old PGP into it.
It’s a shame that encryption things like this are not well integrated into email programs. It’s more of a copy/paste operation in order to get encrypted mail working. Especially with web-based mail like gmail.