Here's the explanation of what it means for a program to be a GNU
package:

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Calling a program GNU software means that its developers and the GNU
project agree that "This program is part of the GNU project, released
under the aegis of GNU"--and say so in the program.

This means that we normally put the program on ftp.gnu.org (although
we could instead refer to the developer's choice of ftp site).

This means that we put the official pages describing the program on
the www.gnu.org, specifically in /software/PROGRAMNAME.  Whenever you
give out the URL for the package home page, you would give this
address.  (It is ok to put more informal pages about secondary issues,
such as discussion meant for people who want to help develop the
package, and pages that access data bases, on some other site.)

It means that the developers agree to pay some attention to making the
program work well with the rest of the GNU system--and conversely that
the GNU project will encourage other GNU maintainers to pay some
attention to making their programs fit in well with it.

Just what it means to make programs work well together is mainly a
practical matter that depends on what the program does.  But there are
a few general principles.  Certain parts of the GNU coding standards
directly affect the consistency of the whole system.  These include
the standards for configuring and building a program, and the
standards for command-line options.  It is important to make all GNU
programs follow these standards, where they are applicable.

Another important GNU standard is that GNU programs should come with
documentation in Texinfo format.  That is the GNU standard documentation
format, and it can be converted automatically into various other formats.
You can use DocBook format or another suitable format for the documentation
sources, as long as you verify that converting it automatically into
Texinfo gives correct and reasonable results.

If a GNU program wants to be extensible, it should use GUILE
(http://www.gnu.org/software/guile/guile.html) as the programming
language for extensibility--that is the GNU standard extensibility
package.  If the program doesn't use GUILE today, at least there
should be a firm plan to support it in the future.

A GNU program should use the latest version of a license that the GNU
Project recommends--not just any free software license.

A GNU program should not recommend use of any non-free program, and it
should not refer the user to any non-free documentation for free
software.  The need for free documentation to go with free software is
now a major focus of the GNU project; to show that we are serious
about the need for free documentation, we must not contradict our
position by recommending use of documentation that isn't free.

Occasionally there are issues of terminology which are important for
the success of the GNU project as a whole.  So we expect maintainers
of GNU programs to follow them.  For example, the documentation files
and comments in the program should speak of Linux-based GNU systems or
GNU/Linux systems, rather than calling the whole system "Linux", and
should use the term "free software" rather than "open source".

Deciding that a program is GNU software does not necessarily require
transferring copyright to the FSF; that is a separate question.  If
you transfer the copyright to the FSF, the FSF will enforce the GPL
for the program if someone violates it; if you keep the copyright,
enforcement will be up to you.