[Zope] GPL question

[Alan Snyder]

Hi everyone. I have a sort-of off-topic question about zope, plone and
the GPL. If I write an application that sits on top of plone by using
the CMF and the plone components and such, how must my portions be
licensed? I've not written anything yet but just to be sure, if i
write plone stuff that calls plone and cmf and zope functions and
objects, must that also be GPL'd. I was under the assumption that
calling GPL software didn't required making your stuff GPL but if you
were to write code inside a module that is already gpl'd - like
expanding it, then you'd obviously have to gpl it because you're
modifying code - not writing new code. Also, I know the zope license
is ZPL and more commercially friendly than the gpl, but plone is gpl
so obviously there is a boundary there that is being enforced
correctly.

TIA