If you build a system running on GPL software and supply that system to other people/corporations, you must also offer them the source code, including any modifications you made to it (which would include, for example, any custom Linux device drivers for your hardware). It has to be offered in a complete, usable form, including any build scripts etc, so they are able to actually build the software from source and run it on the machine.
So John Deere are obligated to enable people to independently build and get "John Deere Linux" running on their tractors. What they are not obligated to do is supply the source to any of the custom applications that run under that system and presumably are what do the actual work of running the machines. That comes under "mere aggregation", ie closed source software can share the same media and run under a GPL operating system without being obligated to follow the GPL itself.
The distinction is usually made on the basis of whether the closed source code links to the GPL code, or just interacts via an API. If it links it is considered a derivative work, and should be released as GPL.