GPL & Subclassing

[Jason Harris]

Hello,

I apologize if this has been asked before.  I'd like to include a single
file from your implementation of AppKit in a Cocoa project (modified, of
course, to work with the Cocoa headers and without a gnustep library
around).  I'd like to subclass this file to add functionality.  My project
is closed source.

Can I do this without violating the terms of the GPL?  I'm currently rolling
my own implementation and I'd like to avoid reinventing the wheel if
possible.

Thanks for the help!

Jason