In order to have a viable claim, you would have to prove, among other things that the 2 day delay caused your father's inability to move. Medicine is not an exact science and sometimes other than rule things out, it is difficult to make an accurate diagnosis. Consult with a local experienced medical malpractice attorney and have them investigate.
To have a basis for a malpractice lawsuit, you need more than a misdiagnosis or mistreatment of a disease or an injury. It must be proved that the malpratice caused an injury. There is no specific treatment for GBS, and certainly no treatment that would prevent it from progressing to paralysis. Therefore, it's hard to prove damages as a result of such a misdiagnosis. Nothing in your question suggests that anything happened to your Dad that would not have happened anyway. Most people with GBS get better eventually. That said, I know of at least one malpractice case where the patient had GBS and had trouble breathing. The docs waited too long to put the patient on a ventilator and she suffered brain damage as a result. That was a strong case.
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As David said one of the main issues is a two delay in Guillan Barre is not significant. It is an autoimmune disease that needs to run it and supportive care is given. While they certainly did not diagnose your father correctly, even if they had, the treatment is supportive care and ventilatory assistance when needed. This is a horrible disease, but usually goes away with time. You always have the option of discussing with a malpractice attorney.