Setting umask for the overall Gnome session

Hi,

I want to set the umask of a user (or all) within his gnome session. I'm 
using Gnome Shell 3.18.3 without wayland, on latest archlinux.

I tried standard places of setting umask, such as /etc/login.defs, 
/etc/profile, /etc/pam.d/system-auth with "session optional 
pam_umask.so, but nothing works. On tty sessions, the user have the 
defined umask, but on Gnome, he always have 0022 no matter what I define.

In the list archives, I only found this discussion, from 2005, but I 
doubt this is a good solution today:
https://mail.gnome.org/archives/gnome-list/2005-January/msg00004.html

I also tried to get help from other users on the internet, but didn't 
get much answers:
https://unix.stackexchange.com/questions/254378/how-to-set-umask-for-the-entire-gnome-session


Is there a known way to do so? Do you have other suggestions?
If this is not the good place to ask, do you know where I could ?


By the way, the use case is that we share files between users of a 
group, and I want any file created from Gnome with rw-rw-r-- 
permissions, so that other users from this group can edit them later. So 
I thought that I'd use a 0002 umask. I checked that Nautilus is creating 
files with these permissions if I open it from a terminal where I have 
changed the umask before. Now I need to change the umask for the overall 
gnome session.

-- 
dr4Ke.