If your physician recommends gammaglobulin, take his/her advice. It is an expensive medication not prescribed without good reason, and in many cases means the difference between life or death.
Generally, Gamma Globulin is used to boost the immune system but only for a temporary effect. Gamma Globulin is injected to patients who don't produce enough antibodies (also known as immunoglobulins) of their own.
I'm not a doctor, but my aunt was familiar with this solution, told me the basics. :)