What’s really telling is the high number of people who work in Chicago and live in Gary Indiana. From what I understand, Gary is a one hour train ride to Chicago. You can have basically the same commute in the Chicago area with a lot more taxes and hassle.
I don’t have a recent South Shore Line schedule at hand...but Gary should be less than an hour’s direct ride from Chicago, objectively speaking. I suspect many of the “Gary” riders actually live in nearby Merrillville and Schererville (which exploded as people escaped Gary’s collapse in the early 1970s) and drive to a nearby stop like Hammond or Miller, both of which straddle Gary.