As you have worked out, there are two meanings of "give somebody a look".
The "I look at him" usage entails more than just looking at someone. It involves looking at someone - specifically meeting their gaze - using a specific expression which conveys a particular message to them. It may convey disapproval, as in the example you gave, or surprise, or joy etc etc. Sometimes the message is specified, as in "really dirty look"; sometimes the message is clear only from the context, as in "I thought he was going to tell the policeman what we had been doing, so I gave him a look. That shut him up."
The "I gave him a look at the car" usage does not imply that you "made" him look at the car, rather that you "allowed" him to look, or "invited" him to look at the car. There is no element of compulsion implied.
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