I believe it is a genetic disorder. The terms GSID(Genetic Sucrase Isomaltase Deficiency) and CSID(Congenital Sucrase Isomaltase Deficiency) are interchangeable.
The cause of the symptoms is a lack of the enzyme 'Sucrase' which breaks down the disaccharide 'Sucrose' into monosaccharides. Monosaccharides can be absorbed into the body in the intestine, disaccharides cannot. The undigested sugars(disaccharides) ferment in the intestine causing pain, gas, bloating leading to diarrhoea.
Similarly a lack of the enzyme 'Isomaltase' which breaks down starch into smaller parts for absorption causes the same symptoms.