ecthyma; prurigo
Medical term for a skin condition; ecthyma is a deep bacterial infection, while prurigo refers to itchy nodules. The Japanese term may historically cover both, but modern usage is rare and largely replaced by more specific dermatological vocabulary.
Old medical books list a disease called 雁瘡.
がんがさ is an old dermatological term that is hardly used today.
The derivation is uncertain. The kanji 雁 (wild goose) and 瘡 (sore, skin eruption) may be a descriptive or metaphorical compound, but the exact origin is unclear.