A question about the GNU General Public License

Just to set the setting, Client A has contracted Agency X (which is in fact not us) do provide a backend app for their ecommerce system. Agency X has contracted Developer M to create the app. Developer M has decided to do this using GNU as a platform.

Now here's the question: Are the additional modules/modifications made by M covered by the GNU GPL? Specifically, do they have to provide the source code of those modules/modifications if asked?