Catalyzes the hydrolytic deamination of guanosine, producing xanthosine and ammonia. Deaminates exclusively guanosine and 2'-deoxyguanosine but no other aminated purines, pyrimidines, or pterines. Deamination of guanosine by GSDA is the only source of xanthosine production in Arabidopsis. guanosine + H(+) + H2O = NH4(+) + xanthosine Localizes predominantly to the nucleus. Expressed in roots, leaves, flowers and siliques. No visible phenotyper under normal growth conditions. Belongs to the cytidine and deoxycytidylate deaminase family.