http://www.gigabyte.com/support-downloads/faq-page.aspx?fid=816
"1 long beep and 2 short beep:Graphic card error"
If it doesn't work in either of the slots even though you've connected all of the power cables, the GPU might be faulty. What GPU did you have before? Did you uninstall all of the old drivers with driver sweeper before installing the new card? http://driver-sweeper.en.softonic.com/