You didn't do your research very well?
4. You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.
...
6. Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions. You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.
(emphasis mine).
Check with a lawyer. Your additional conditions might count as sublicensing, in which case you're not allowed to. And more obviously, your additional contract might count as imposing further restrictions. The GPL is clearly trying to avoid exactly the situation where someone tries to get around it by making people sign another contract first. It is quite explicit about that.
IANAL, but I believe that there exceptions to what counts as "distribution" in certain work situations. For instance if you have proprietary code mixed with GPLed code, nobody can make you distribute it. And it is OK to put that code on a new employee's machine because you're not actually distributing it to them, they are instead becoming part of the legal entity which is your company.
To get it straight as to what is allowed and what is not here, you really need to talk to a lawyer. Because it comes down to what kind of distribution counts as distribution under copyright law. And that isn't always very simple.
However once you've distributed under copyright law, then you either have to release your own code under GPL or violate copyright law yourself. And releasing under GPL does not allow you to try to release it kinda, sorta, but not really freely to that person.
Cheers,
Ben
