@vijay26 I believe Gadolinium is used only for MRI scan and not for DAT Scans. Gadolinium has metallic properties that can be "seen" by the MRI magnet. By the way, Gadolinium is hard on some patients kidneys.
In contrast the radioactive isotope Iodine 137 is "seen" by the Nuclear Medicine DAT Scan because the scanner is like a large geiger counter reading the radiation flowing through your body. There are other radioactive isotopes with various levels of radiation to produce the best image depending on the type of scan. Your doctor and radiologist should be able to guide you to determine other options if you are allergic to Iodine 137.
Hope this helps, Joe