Ginger at 1000-2000 mg in one study was found to reduce gastric contractions, nausea and blood plasma levels of vasopressin, a peptide hormone that can cause nausea. In this study, injection of vasopressin was found to induce nausea, so it seems that the primary physiological effect of ginger is the suppression of this hormone and the effects it has on causing stomach contractions and subsequent nausea. This theory is based on this report, although the authors did not conclusively say that this was the effect. Read it for yourself and the reference provided for vasopression, where it one of its activities is given as cause of nausea.

Regarding the concentration, if you could approach the levels used in the study, you should use about twice the fresh weight of ginger. This is a general guideline for the use of fresh versus dried herbs. So the upper limit would be 4 grams of fresh ginger or 4,000 milligrams.

I wasn't aware of this herbal remedy for nausea before. Thanks for bringing this question forward. I learn something every day.

Ginger contains phenols that relaxes the stomach muscles and has sedative effect on the tissues of the stomach. It will also decrease the over activity of the stomach which helps in reducing nausea and pain. Phenols also help in increasing movement inside the intestine thus allowing digested food and toxins through the digestive tract. You can use ginger in any kind of form but it would be preferable to use those that are readily available to save time.