GNU license for a project

[Hazael Delgado Salazar]

Hello.

I'm working with a team that is developing a web aplication with HTML,
PHP and MySQL.

Finished the application the plan is share it with other persons as
free software, in order that any person who owns it, can make any
improvement, or any correction BUT making available the new/corrected
module for the other owners.

I read the GNU license's terms and conditions in
http://www.gnu.org/licenses/gpl.html#SEC1

Among other things, under point 1) says that I can share or distribute
programs I recieve, giving the same rights that I got when I recieved
it, and publishing a copy of the copyright notice and disclaimer of
warranty...

under point 2) says that I can modify copies of the program and
distribute them under the section's 1) terms and the a) b) c) terms of
the point 2)


Well, I got a little confused about the topic.So, my question is:
*Within the GNU licenses topic*, what else do I need in order to can
regulate the "posting" of additions/corrections to the system is being
developed?
In other words: Is it enough to us only apply a GNU license to the web
application?
do we need to add "additional terms" or something else?

Thanks in advance.