When managing high fasting levels of gestational diabetes with metformin and allergies to insulin, there are alternative strategies you can try. Firstly, consult your healthcare provider for personalized advice. They may suggest adjusting your diet and exercise routine to help regulate blood sugar levels. Additionally, they might recommend alternative medications or treatments that are suitable for your specific situation. It's crucial to closely monitor your blood sugar levels and follow the prescribed treatment plan. Regular check-ups with your healthcare provider will ensure that your condition is properly managed throughout pregnancy.