How do I sign an apk with a private key I made with GPG?

I very much doubt that GPG generates keys that could be used by jarsigner. It might be possible to write a converter to do this, but it would be far less work to just bite the bullet and generate a new key. The command to do this is simply

keytool -genkey -alias mynickname -validity 20000 -keystore ~/.android/my-keystore

(p.s. make a backup of the key and make very sure you don't forget either the keystore password or the key password. There are far too many sad stories of people who've put apps on the market and then forgotten or lost the password.)