Do you have GDD, MCSS, or both? MCSS is atleast as nebulous a condition (actually much more so) than GDD, and I believe it is probably a not so dissimilar concept of an immune system reaction to foreign antigens. Treatment is different so sorting them out is somewhat important. MCSS is just as it sounds- when someone experiences one type, a few types, or many types of chemicals they react to it. They can ofcourse react to a GBCA as one of the chemicals. MCSS as a whole though results in symptoms that I describe as B symptoms for GDD, which include: head-ache, loss of energy, nausea, and vomiting. These symptoms can last for quite a while days or weeks (at times months+ longer), rather than hours, which most (perhaps all) people experience with their body experiencing chemicals it does not like. So receiving a GBCA and experiencing a headache lasting for months, perhaps combined with loss of energy and nausea may actually represent MCSS and not GDD.
The primary treatment for both is the same: don't receive another GBCA again. Probably not 100% essential for MCSS, 100% essential for GDD.
GDD most often (meaning not all) should experience classic GDD symptoms, the most distinctive are burning skin pain, boring bone pain, and sharp pins and needles (or similar sensation) - particularly in a lower arm+hand and lower leg+foot distribution.
A critical part of treatment for GDD is chelation with a strong chelator for Gd (at present the only available is DTPA). Interestingly, although not essential for MCSS, they may also achieve some benefit from chelation, as decreasing the amount of Gd retained in the body may lessen the continued impulse for MCSS reaction. MCSS should not carry with it the potential for serious long term bad consequences, as GDD. An extended hypersensitivity protocol is important for GDD during chelation, and a similar but lesser strategy for MCSS. Some autoimmune treatment likely is useful for both... starting probably with LDN.
MCSS probably experiences reactions to multiple other chemicals besides GBCA (making the MCSS diagnosis). The reactions are almost always not so long-lasting and so potentially serious as with GDD (just type B symptoms, generally). GDD may be much more specific to Gd, but probably most individuals react to the family of heavy metals to some extent.
So if you have GDD but react to also a whole host of other unrelated chemicals and drugs, you probably have combined GDD and MCSS - which doesn't make it worse, just means you have to avoid more different chemicals.