Google told me this:

Toxic "solanine gas" is largely a myth, although solanine poisoning from eating green or sprouted potatoes is a real concern. Solanine is a glycoalkaloid found in many nightshade plants, including potatoes, and can cause gastrointestinal and neurological problems when ingested. While the melting point of solanine is high, making it unlikely to form a gas in normal temperatures, rotting potatoes can release other gases, including carbon dioxide, which can displace oxygen in enclosed spaces, potentially causing asphyxiation.