How to use service callout for google access token

[RitaKuo]

I am designing a proxy for this API because the API has this Authorization:

curl -X POST -H "Authorization: Bearer $(gcloud auth print-access-token)" \
-H "Content-Type: application/json"

I set service callout、Extract token、AssingMessage to achieve the goal。But I keep getting the 401 message of token error.("message": "Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.")
Is there any wrong setting in my service callout?

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

<ServiceCallout async="false" continueOnError="false" enabled="true" name="SC-auth">

  <DisplayName>SC-auth</DisplayName>

  <Request>

    <IgnoreUnresolvedVariables>false</IgnoreUnresolvedVariables>

    <Set>

      <Headers>

        <Header name="Content-Type">application.json</Header>

        <Header name="Authorization">bearer</Header>

      </Headers>

      <FormParams>

        <FormParam name="grant_type">refresh_token</FormParam>

        <FormParam name="refresh_token">{request.formparam.a}</FormParam>

      </FormParams>

    </Set>

  </Request>

  <Response>accessTokenResponse</Response>

  <Timeout>30000</Timeout>

  <HTTPTargetConnection>

    <URL>https://www.googleapis.com/auth/cloud-platform</URL>

    <Authentication>

      <HeaderName ref="Authorization">STRING</HeaderName>

      <GoogleAccessToken>

        <Scopes>

          <Scope>https://www.googleapis.com/auth/cloud-platform</Scope>

        </Scopes>

      </GoogleAccessToken>

    </Authentication>

  </HTTPTargetConnection>

</ServiceCallout>