Is my mobo compatible with the r9 280 gpu

it should work, PCI Express is backwards compatible, the card is PCI express 3.0. The mother board appears to be PCI Express 1.0. So yes it will work but not as well as it can. Also you would want to make sure it fits in your current case, the card is going to be over 10" long. 10.32" on the Sapphire model i was specing for this.
 

Board look like it is pcie 2.0 http://www.gigabyte.com/products/product-page.aspx?pid=3773#ov
so there should be next to no performance loss
 


(All PCI Express slots conform to PCI Express 2.0 standard.)

that's the site i looked at, totally missed that part in the ( )


Good catch thanks