Ubuntu – Stuck at 800×600 display resolution Ubuntu 14

After dual booting Win 10/Ubuntu 14, Windows displays at 1366 x 768 with 10 display modes available.
Ubuntu has no options other than a "built-in" 800 x 600 low res output. The driver is the latest available for the AMD Radeon R6. This is for an HP Pavilion Notebook.

Research (mostly about nvidia cards) suggests use of xrandr and cvt commands in terminal which I've attempted without joy with a file attached containing outputs from various commands.

Notably,"size of gamma" and the name of the video device (ie, VGA) can't be found by xrandr.
What can be tried to access and use a resolution of 1368×768?
What is "gamma" and why can't xrandr find it?

Here is further information including commands/outputs.

Computer: an HP Pavilion Laptop 64bit dual-booted Windows 10/Ubuntu 14.04 LTS.
Graphics card: AMD Radeon R6, chip: Ox9874
Driver:AMD/ATI display driver wrapper from xserver-xorg-video-ati(open source, tested)--recommended.

OUTPUT from $>sudo lshw -C video:

 *-display UNCLAIMED     
       description: VGA compatible controller
       product: Advanced Micro Devices, Inc. [AMD/ATI]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 1
       bus info: pci@0000:00:01.0
       version: c5
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list
       configuration: latency=0
       resources: memory:e0000000-efffffff memory:f0000000-f07fffff ioport:f000(size=256) memory:ff700000-ff73ffff memory:ff740000-ff75ffff

OUTPUT from $>xrandr:

xrandr: Failed to get size of gamma for output default
Screen 0: minimum 800 x 600, current 800 x 600, maximum 800 x 600
default connected primary 800x600+0+0 0mm x 0mm
   800x600        75.0*  

OUTPUT using cvt with known max resolution from WIN 10 where 10 possible display modes exist….

~$> cvt 1366 768 60"

# 1368x768 59.88 Hz (CVT) hsync: 47.79 kHz; pclk: 85.25 MHz
Modeline "1368x768_60.00"   85.25  1368 1440 1576 1784 768 771 781 798 -hsync +vsync

~$ lspci| grep VGA
00:01.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Device 9874 (rev c5)

After attempting to add a newmode to xrandr and testing this output occurs;

~$ xrandr --newmode  "1368x768_60.00"   85.25  1368 1440 1576 1784  768 771 781 798 -hsync +vsync
xrandr: Failed to get size of gamma for output default
X Error of failed request:  BadName (named color or font does not exist)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  16 (RRCreateMode)
  Serial number of failed request:  19
  Current serial number in output stream:  19

~$ xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 800 x 600, current 800 x 600, maximum 800 x 600
default connected primary 800x600+0+0 0mm x 0mm
   800x600        75.0* 
  1368x768_60.00 (0x233)   85.2MHz
        h: width  1368 start 1440 end 1576 total 1784 skew    0 clock   47.8KHz
        v: height  768 start  771 end  781 total  798           clock   59.9Hz

When attempting to use >xrandr to --addmode these error messages occur:

xrandr --addmode default 1366x768_60.00
xrandr: Failed to get size of gamma for output default
xrandr: cannot find mode "1366x768_60.00"

additional attempt to get xrandr to accept a newmode and the resultant output:

~$ xrandr --newmode  "1368x768_60.00"   85.25  1368 1440 1576 1784  768 771 781 798 -hsync +vsync
xrandr: Failed to get size of gamma for output default
X Error of failed request:  BadName (named color or font does not exist)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  16 (RRCreateMode)
  Serial number of failed request:  19
  Current serial number in output stream:  19