If you want to retain control of the software written under contract, you must make sure that your contract doesn't include the term "Work for Hire".
Many companies will include a "Work for "Hire" clause in cosulting or subcontract agreements. "Work for Hire" means they completely own the new software as if it was created by their own employee.
Since the new software is a derivative work of something already licensed by the GPL, the new software must remain under the GPL.
However as someone already mentioned, if they don't distribued the software outside the company they don't have to share the source, not even with you after the job is completed.
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If you want to retain control of the software written under contract, you must make sure that your contract doesn't include the term "Work for Hire".
Many companies will include a "Work for "Hire" clause in cosulting or subcontract agreements. "Work for Hire" means they completely own the new software as if it was created by their own employee.
Since the new software is a derivative work of something already licensed by the GPL, the new software must remain under the GPL.
However as someone already mentioned, if they don't distribued the software outside the company they don't have to share the source, not even with you after the job is completed.