Hi I am thinking of releasing a plugin, it seems that WordPress pushes for all plugins to be released as GPL. I have no problem with that, but I've noticed that Gravity forms plugin is GPL + requires a licence key. Is it ok to issue a plugin under the GPL licence but then require a licence key for it to work?
If a plugin is GPL, you have the right to modify, tweak, rebuild, and otherwise hack the plugin at your leisure. You also, under the GPL, have the right to redistribute your modified version and, ultimately, the unmodified version as well.
The Gravity Forms license key is for support and updates. If you don't have a license, or if your license expires:
Gravity Forms does not require the license key for the plugin to work, just for it to communicate with their servers to retrieve updates. This is perfectly acceptable behavior for a GPL plugin.
Can you require a license key?
If you read the terms of the GPL, you are required to provide all of the "installation instructions" that are required for proper function of the system. From the license itself:
So no, you cannot release a plugin under the terms of the GPL and require a license key for the plugin to work.
I can't speak intelligently as to whether or not releasing a crippled version (or completely non-functional version) of a plugin is legitimate under the GPL license. You would want to research that, but I suspect it's a problem.
Gravity Forms is actually providing a download and update service for a fee, and the license key has the sole purpose of being able to synchronize with their servers and automatically update the plugin. But the plugin itself isn't crippled in any way if you don't have a license key (it's fully functional).
So under the GPL it's actually completely legal for someone to pay for that service and redistribute the version they've downloaded for free. It might be against their terms of service and they could cancel your subscription... And that'd be a pretty nasty thing to do IMO... But it's legal.