This article will describe a way on how to change/set a primary display on CentOS/RHEL 7 with dual monitors and GNOME. The easiest and perhaps recommended way is to use GUI and navigate to Applications->System Tools->Settings->Displays.
The current primary screen is highlighted by a black top bar. To change you primary display simply drag-n-drop the black top bar from one screen to another:
xrandr. First get all your connected screens:
$ xrandr | grep -w connected DVI-D-1 connected 1680x1050+0+0 (normal left inverted right x axis y axis) 474mm x 296mm HDMI-1 connected primary 1920x1080+1680+0 (normal left inverted right x axis y axis) 598mm x 336mm
From the output above we can see that there are two connected screens DVI-D-1 and HDMI-1 where HDMI-1 is connected as a primary screen. To set for example DVI-D-1 as a primary display execute the following linux command:
$ xrandr --output DVI-D-1 --primary
The above command will switch you primary displays. However, this setting will not persists after reboot unless you add you command to you ~/.bashrs file.