I have tried now two more combinations (in /etc/environment to stabilize gnome
session):

MESA_GL_VERSION_OVERRIDE=3.0
MESA_GL_VERSION_OVERRIDE=3.1 

Up to GL 3.0, everything works fine and stable, glmark2 and gnome session. 

With 3.1 or any other higher glmark2 shows only black content on the window
opened when I use MESA_GL_VERSION_OVERRIDE=3.1 glmark2 in same session that I
openned using /etc/environment. 

If I change to: 

MESA_GL_VERSION_OVERRIDE=3.1 in /etc/environment, then also gnome session
(after login in gdm) got "gdm background" with screen freezed video.