No additional software is required on a full Slackware 14.1 install.
Slackware 14.1 ships with KDE 4.10.5.

Prerequisites

If you don’t already have a key pair (secret and public key), then your first order of business is to create one. The preferrable option is to use the CLI with the command:

gpg --gen-key

KDE provides two graphical tools for working with GPG, namely KGpg and Kleopatra.  KGpg will walk you through the initial setup using an interactive wizard and subsequently create the needed key pair and config files (do not use Kleopatra for your initial setup).
If you’re importing an existing private key then make sure to change the default trust level afterwards.