So you're using windows and have Kleopatra installed.
IIRC, you should be able to encrypt files with Kleopatra aswell, that being said it has been a long time since i used this tool, so i might be mistaking.

An other option would be to just install openssl. It's open source and the sourcecode can be downloaded here: https://www.openssl.org/source/
Or if you prefer a binary, so you don't have to compile it yourself, you can look over here: https://wiki.openssl.org/index.php/Binaries