Got an email from GMF yesterday with the following in it. Looks like you can extend your lease by one month without even contacting them:
One-month automatic lease extension - If you are unable or choose not to return your vehicle, we will process a one-month extension to allow you additional time. No need to contact us – if we don’t hear from you 10 days after your original termination date, we will automatically extend your lease for one month. You will receive notification of the extension at this email address. Later, if more time is needed, we will work with you on a solution that meets your needs.