While using startx (or other alternative), gnome respond by giving a black screen following by a mouse cursor then shutting down without error. I did paste to the mailing list my log...Here my xinitrc:

export GDK_BACKEND=x11
exec gnome-session
#exec startxfce4
#export XDG_CURRENT_DESKTOP=GNOME-Classic:GNOME
#export GNOME_SHELL_SESSION_MODE=Classic
#exec gnome-session --session=gnome-classic

here foo.log:

X.Org X Server 1.20.2
X Protocol Version 11, Revision 0
Build Operating System: Linux Arch Linux
Current Operating System: Linux m81 4.14.70-gnu-1-lts #1 SMP Wed Sep 19 08:04:57 UTC 2018 x86_64
Kernel command line: BOOT_IMAGE=/vmlinuz-linux-libre-lts root=UUID=abdec431-03ba-4415-9c74-c808a88cd1cc rw quiet
Build Date: 16 October 2018 03:15:21PM

Current version of pixman: 0.34.0
Before reporting problems, check http://wiki.x.org
to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
(++) from command line, (!!) notice, (II) informational,
(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/home/j2lapoin/.local/share/xorg/Xorg.0.log", Time: Sat Oct 20 12:37:20 2018
(==) Using system config directory "/usr/share/X11/xorg.conf.d"
xf86EnableIOPorts: failed to set IOPL for I/O (Operation not permitted)
resize called 1280 1024
The XKEYBOARD keymap compiler (xkbcomp) reports:

Warning: Unsupported high keycode 372 for name <I372> ignored
X11 cannot support keycodes above 255.
This warning only shows for the first high keycode.

Errors from xkbcomp are not fatal to the X server
The XKEYBOARD keymap compiler (xkbcomp) reports:

Warning: Unsupported high keycode 372 for name <I372> ignored
X11 cannot support keycodes above 255.
This warning only shows for the first high keycode.

Errors from xkbcomp are not fatal to the X server
xinit: connection to X server lost

waiting for X server to shut down (II) Server terminated successfully (0). Closing log file.