Disclaimer: I am not a lawyer, this is my understanding as a long time member of the open-source community.
First things first, if your employer owns the copyright to all the code they are not bound by the GPL, the GPL only binds them if they are redistributing GPL code owned by others.
Secondly, (unless you have previously made a physical distribution under option 6b) there is nothing in the GPL that requires offering the code to everyone, so even if the code is a derivative work of other GPL code, putting it behind a registration form should not be a problem from that angle.
The one area where you may have to be careful is if the work is a derivative of other GPL code and you are distributing the work in binary form. In this case you must comply with one of the options listed in section 6 of the GPL.
Options a, b are only relavent to physical media distribution. Option c is only relavent if the distribution is noncommercial and the place you got the code from used option b. Option e is only relavent for distribution through peer to peer networks (and mostly references option d anyway).
So that leaves you with option d.
d) Convey the object code by offering access from a designated place (gratis or
for a charge), and offer equivalent access to the Corresponding Source in the
same way through the same place at no further charge. You need not require
recipients to copy the Corresponding Source along with the object code. If the
place to copy the object code is a network server, the Corresponding Source may
be on a different server (operated by you or a third party) that supports
equivalent copying facilities, provided you maintain clear directions next to the
object code saying where to find the Corresponding Source. Regardless of what server
hosts the Corresponding Source, you remain obligated to ensure that it is available > for as long as needed to satisfy these requirements.
I would interpret this as it being ok to put the source and binaries behind the same registration wall. But not to distribute the binaires openly while putting the source behind a registration wall.