Xcode AdMob's GAD_SIMULATOR_ID undeclared identifier error
Starting from the SDK version 7.0.0 you no longer need to precise GAD_SIMULATOR_ID so as to run test ads in the simulator. It is now done automatically.
Please, have a look at the banner example here: it is precised in the comments.
EDIT:
From SDK 7.0 to 7.1 you can use kDFPSimulatorID
Starting from the SDK 7.2 you can use kGADSimulatorID