When your only navaids are an ADF, a driftsight and a astrocompass the ADF starts to look pretty good. Nobody had thought of GPS then.
I made no comment about the accuracy of ADF, except in the overhead, when you can be pretty certain that you know where you are. GPS may be good, but it lacks the certainty.
The original question asked if GPS was a substitute for an ADF, it is not. It does make a good alternative, and is even better if one compliments the other.