Not gold unfortunately. The Color is that of 22K and Guess has never manufactured anything in high karat gold, or even low karat to my knowledge. People go to the other "G" brand for that.....Gucci. Additionally, if it were gold it would definitely be hallmarked as its a modern piece not manufactured privately. Im not sure how your father tested it, but if he used the acid test, for a true reading you will have to cut into the ring with the side of a small file to expose the core alloy and place the acid directly on the cut.
Regardless its a good looking piece so enjoy it, but alas it can't be resized unfortunately.