Please visit https://www.authy.com/install from your phone to install the Authy app again on your new phone.
Once installed, it should prompt you for your phone number (please enter the number registered with your Gemini account). Authy will then send you an SMS text message with a code to enter into the Authy app. Receiving the code confirms you have access to the phone number entered; the Gemini token should re-appear within Authy shortly thereafter.
If these steps do not resolve your issues, the SMS never comes, or Gemini is not listed in the Authy app, please visit https://www.authy.com/phones/reset to reset your Authy account.