Veterinary questions? Ask a Vet for Answers ASAP
You are through to Dr Coles. I am a UK based vet. I am sorry to hear you are having problems with your gecko. Often they have their mouth wide open as a defensive mechanism or when stressed. I would double check that there is nothing stuck inside her mouth then carefully check her eyes, nose and ears for any abnormalities. It is possible there could be some shed stuck over her nostrils and causing difficulty breathing. If this continues, I would advise calling the emergency vets to have her seen tonight as there is likely something causing severe discomfort or respiratory distress for her to have her mouth wide open. Respiratory infections are possible, but sometimes Geckos will hold their mouth open when in pain as well. I would also check that nothing in the enclosure has changed and ensure that your temperatures are adequate (not too hot).
Leopard geckos do not hibernate or get the cold. They can get respiratory infections but usually these are bacterial/viral and secondary to another condition causing immune suppression. I would definitely advise having him checked ASAP.