There are two types of "credentials" in OAuth2 process: access token (its lifetime is short and determined by expires_in field of the OAuth2 server response) and refresh token which is virtually permanent unless user revokes granted access. When you possess refresh token you can get valid access token.
Refresh token is returned upon first auth request with parameter access_type=offline. All you need is to store obtained refresh token securely and use it to request for access token when necessary.
Read more in the document https://developers.google.com/accounts/docs/OAuth2WebServer