From OMIM
GM1-gangliosidosis type III (GM1G3) is an autosomal recessive lysosomal storage disorder characterized by neurodegeneration and mild skeletal changes. Age at onset ranges from 3 to 30 years. The disorder is less severe than GM1-gangliosidosis types I and II (230600). Type III shows extreme clinical variability, with some patients having only focal neurologic signs, such as dystonia, and others having more severe involvement with extrapyramidal signs and mental retardation. GLB1 enzymatic activity usually ranges from approximately 4 to 10% of control values (Suzuki et al., 2001).  http://www.omim.org/entry/230650

From MedlinePlus Genetics
GM1 gangliosidosis type II occurs in one of two forms: the late infantile or the juvenile forms. Children with type II develop normally early in life, but they begin to show signs and symptoms of the condition around the age of 18 months (late infantile form) or 5 years (juvenile form). Individuals with GM1 gangliosidosis type II experience developmental regression but usually do not have cherry-red spots, coarse facial features, or enlarged organs. Type II usually progresses more slowly than type I, but it still  shortens life expectancy. People with the late infantile form typically survive into mid-childhood, while those with the juvenile form may live into early adulthood.

People with GM1 gangliosidosis type I can lose their vision due to clouding of the clear outer covering of the eye (the cornea) and the breakdown of the light-sensing tissue at the back of the eye (the retina). Affected individuals also develop a red area in the eye known as a cherry-red spot. In some cases, affected individuals have distinctive facial features that are described as "coarse," enlarged gums (gingival hypertrophy), and an enlarged and weakened heart muscle (cardiomyopathy). Individuals with type I usually do not survive past early childhood.

GM1 gangliosidosis type III is the adult or chronic form of the condition, and this is the mildest form. The age at which symptoms first appear varies in people with GM1 gangliosidosis type III, although most affected individuals develop signs and symptoms in their teens. The characteristic features of this type include involuntary tensing of various muscles (dystonia) and abnormalities of the spinal bones (vertebrae). Life expectancy varies among people with GM1 gangliosidosis type III.

The signs and symptoms of the most severe form of GM1 gangliosidosis, called type I or the infantile form, usually develop by the age of 6 months. Infants with this form of the disorder typically appear normal until their development slows and the muscles used for movement weaken. Affected infants eventually lose the skills they had previously acquired (developmentally regress) and may develop an exaggerated startle reaction to loud noises. Over time, children with GM1 gangliosidosis type I develop an enlarged liver and spleen (hepatosplenomegaly) and skeletal abnormalities. Affected children often  have seizures and profound intellectual disability. 

GM1 gangliosidosis is an inherited disorder that destroys nerve cells (neurons) in the brain and spinal cord. This condition can be classified as one of three major types based on the age at which signs and symptoms first appear. However, the signs and symptoms of these three types can overlap, leading some researchers to believe that GM1 gangliosidosis occurs on a spectrum instead of as three distinct types.  https://medlineplus.gov/genetics/condition/gm1-gangliosidosis