Left click on the file you want to decrypt. Enter your passphrase and it will be decrypted. You can also drag an encrypted text file and drop it into KGpg's editor window. It will then ask the passphrase and open the decrypted text in KGpg's editor. You can even drop remote files ! You can also use the File → Decrypt File and choose a file to decrypt.

You can also decrypt the contents of the clipboard with the Decrypt Clipboard menu entry of the KGpg applet. An editor window will show up with the decrypted text.

Copy or Drag and Drop the text you want to decrypt, and click on the Decrypt button. You will be prompted for the passphrase.