A client asked me to write a WP plugin that is just for their website - it won't be on the repository. Basically it contains a form, place-able with a shortcode, and then sends the collected entry data to a third party private API - essentially nobody except my client has any use for the plugin.
Is it mandatory to use the GPL, just because it is a plugin for one specific Wordpress-installation? Is it allowed to use a different license? My problem is, that the client pays for the development and has their customized set-up and doesn't want it to be open source. So is there a different license I could use? If yes which one?