xrandr fails to change screen resolution on Arch Linux

I have a dual-monitor setup in my notebook, which is running Arch Linux with Gnome 3 as Desktop Environment.

My graphic card is Intel HD Graphics 3000 and my external monitor (VGA1) is a Philips 202EL, which best resolution is 1600x900.

I've tried to run the following commands:

$ cvt 1600 900$ xrandr --newmode "1600x900_60.00"  118.25  1600 1696 1856 2112  900 903 908 934 -hsync +vsync$ xrandr --addmode VGA1 1600x900_60.00$ xrandr --output VGA1 --mode 1600x900_60.00

As result, the external monitor screen blinks, but the resolution is not changed. I've tried to run with gtf instead of cvt as well, but with no luck.

Is there something I am missing? Is there any alternative way to fix my monitor screen resolution?