Go to Advanced for the display properties, and check off "List modes your monitor may not support", or something similar, see if you get more options. It seems like the card and/or computer is not detecting your monitor's correct resolution. See if the monitor has a driver for it also, some do to add support for different modes.
I found your problem here: http://www.gigabyte.us/products/product-page.aspx?pid=4147#sp
Integrated Graphics Processor:
1 x D-Sub port
1 x DVI-D port, supporting a maximum resolution of 1920x1200
* The DVI-D port does not support D-Sub connection by adapter.
1 x HDMI port, supporting a maximum resolution of 1920x1200
While the HD 4000 graphics card may be able to support 2560x1600, that motherboard does not.