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It governs the stagnation and accumulation of dampness-toxin. It also treats vomiting, coughing, diarrhea with cold extremities, irritability, abdominal pain, chest pain, and lower back pain.

Evidence:
In the syndrome of Da Jian Zhong Tang (Greatly Building Middle Decoction), there is severe cold in the heart and chest, pain, vomiting, and inability to eat.
In the syndrome of Ling Jiang Zhu Gan Tang (Poria, Ginger, Atractylodes Macrocephala, and Licorice Decoction), the body feels heavy, and the waist is cold. It also states that the area below the waist is cold and painful.
In the syndrome of Ban Xia Gan Jiang San (Pinellia and Dried Ginger Powder), there is dry retching, regurgitation, and spitting of saliva.
In these three formulas, the dosage of Gan Jiang (Dried Ginger) is either four qian or equal to other herbs.
In the syndrome of Ren Shen Tang (Ginseng Decoction), there is a tendency to spit saliva. It also states there is a feeling of痞 (stagnation/blockage) in the heart.
In the syndrome of Tong Mai Si Ni Tang (Unblocking Pulse and Reversing Extremities Decoction), there is diarrhea with undigested food. It also states cold extremities. It also mentions dry retching.
In the syndrome of Xiao Qing Long Tang (Minor Blue Dragon Decoction), there is water-fluid retention in the epigastrium and dry retching. It also mentions coughing.
In the syndrome of Ban Xia Xie Xin Tang (Pinellia Drain the Epigastrium Decoction), there is vomiting accompanied by borborygmus (intestinal rumbling).
In the syndrome of Chai Hu Jiang Gui Tang (Bupleurum, Ginger, and Cinnamon Twig Decoction), there is fullness in the chest and hypochondrium. It also mentions irritability.
In the syndrome of Huang Lian Tang (Coptis Decoction), there is abdominal pain with a desire to vomit.
In the syndrome of Ling Gan Wu Wei Jiang Xin Tang (Poria, Glycyrrhiza, Schisandra, Dried Ginger, and Asarum Decoction), there is coughing and chest fullness.
In the syndrome of Gan Jiang Huang Lian Huang Qin Ren Shen Tang (Dried Ginger, Coptis, Scutellaria, and Ginseng Decoction), there is vomiting and diarrhea.
In the syndrome of Liu Wu Huang Qin Tang (Six-Ingredient Scutellaria Decoction), there is dry retching and diarrhea.
In these nine formulas, the dosage of Gan Jiang (Dried Ginger) is consistently three qian.
In the syndrome of Zhi Zi Gan Jiang Tang (Gardenia and Dried Ginger Decoction), there is slight irritability.
In the syndrome of Gan Cao Gan Jiang Tang (Licorice and Dried Ginger Decoction), there are cold extremities, dry throat, irritability, and vomiting with regurgitation.
In the syndrome of Gan Jiang Fu Zi Tang (Dried Ginger and Aconite Decoction), there is irritability and insomnia.
In these three formulas, the dosages of Gan Jiang (Dried Ginger) are two qian and one qian, representing variations from the four-qian standard.
In the syndrome of Si Ni Tang (Reversing Extremities Decoction), there is diarrhea with undigested food. It also states cold extremities.
In this single formula, the dosage of Gan Jiang (Dried Ginger) is one and a half qian, representing a variation from the three-qian standard.
In the syndrome of Tao Hua Tang (Peach Blossom Decoction), there is diarrhea.
In the syndrome of Gan Jiang Ren Shen Ban Xia Wan (Dried Ginger, Ginseng, and Pinellia Pill), there is incessant vomiting.
In these two formulas, the dosage of Gan Jiang (Dried Ginger) is one qian, representing a variation from the three-qian standard.
Having observed all these formulas comprehensively, the symptoms such as vomiting, coughing, pain, and diarrhea are all caused by the stagnation and accumulation of dampness-toxin.

Mutual Examination:
Sun Simiao stated: "If fresh ginger is unavailable, dried ginger may be used as a substitute." In my observation, Zhang Zhongjing's use of Sheng Jiang (Fresh Ginger) and Gan Jiang (Dried Ginger) has similar main indications but minor differences; Sheng Jiang (Fresh Ginger) primarily treats vomiting, while Gan Jiang (Dried Ginger) primarily treats the stagnation of dampness-toxin. They should not be confused.

Correcting Misconceptions:
Bencao materia medica describes Gan Jiang (Dried Ginger) as extremely hot. Consequently, physicians of the world believe that in Si Ni Tang (Reversing Extremities Decoction), Jiang (Ginger) and Fu Zi (Aconite) are hot medicines, and thus they can warm cold extremities. This is incorrect. Cold extremities are a manifestation of urgent toxin pressure. Therefore, Gan Cao (Licorice) is used as the sovereign herb, while Jiang (Ginger) and Fu Zi (Aconite) are used as assistant herbs. The purpose of using Jiang (Ginger) and Fu Zi (Aconite) is to expel dampness-toxin. Where is the heat?
In Nihonbashi, Byoin Street, Kyoto, there was a man named Kabe Emon, whose shop was called Omiya. His son was thirteen years old. One day, he suffered from diarrhea. By noon, he had lost count of how many times he went to the toilet. His spirit became exhausted and confused. A physician administered Du Shen Tang (Single Ginseng Decoction). By dusk, his extremities became cold. The physician was terrified and increased the dosage of Gan Jiang (Dried Ginger) and Fu Zi (Aconite), but the cold extremities worsened. Other physicians deemed it incurable. I examined him and found his entire body devoid of warmth, his limbs spasmodic against the ground, irritability with loud crying, signs resembling abdominal pain, pulsation around the umbilicus, and tenderness to touch. I said, "This is toxin! Medication can treat it. Whether he lives or dies, I cannot predict; however, treating him now leads to death, and not treating him also leads to death. Since death is inevitable, would you prefer treatment?" The relatives agreed. I then administered Da Cheng Qi Tang (Major Order the Qi Decoction) (one packet weighing twelve qian). After one dose, there was no significant change. I administered another dose. The cold extremities turned into heat. After three doses, his complexion and spirit returned to normal, and diarrhea decreased by half. After ten days of medication, all symptoms subsided. From this, we see that the physician's understanding of this medicine was merely its ability to resolve this toxin. When the toxin resolves, cold extremities become warm, and extreme heat becomes cool. If one attributes the normalization of cold extremities to hot medicines, are Da Huang (Rhubarb) and Mang Xiao (Mirabilite) also hot medicines? The hot, cold, warm, and cool properties of medicines cannot be simply discussed; this clarifies the matter.

Product Examination:
Gan Jiang (Dried Ginger): There are two varieties produced in our country: Gan Sheng Jiang (Dried Fresh Ginger) and Mikawa Gan Jiang (Mikawa Dried Ginger). The variety known as Gan Sheng Jiang (Dried Fresh Ginger) is what my family uses. The variety known as Mikawa Gan Jiang (Mikawa Dried Ginger) is not used by my family.