Start with connecting to firebase:

add google-services.json to android/app

add " classpath 'com.google.gms:google-services:4.3.3' " to android/build.gradle (dependencies)

add " apply plugin: 'com.google.gms.google-services' " to android/app/build.gradle

change minApi to 21

in pubsec.yaml

firebase_core: "^0.5.0+1"

firebase_auth: "^0.18.1+2"

cloud_firestore: "^0.14.1+3"

google_sign_in: ^4.5.3

initialize app before runapp()

Google Login Implementation