This should fix it. Or an easier method can be to add some supportloops, but depending on how and where you want to use the model, maybe its not the best option cause then you get a lot of extra geo.
An another page there mentioning that rigged assets will have 1 smoothing group though, but I'm not a source engine user so I can't tell you surely how it works there. But when I remember back to games with source, I clearly see many hard edges.
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How do you intend to use this model once you have completed it? That has a direct bearing on which method you use to fix it.
Why does it need to be 1 smoothing group?
https://developer.valvesoftware.com/wiki/Smoothing_groups
https://developer.valvesoftware.com/wiki/Hammer_Smoothing_Groups_Dialog
An another page there mentioning that rigged assets will have 1 smoothing group though, but I'm not a source engine user so I can't tell you surely how it works there. But when I remember back to games with source, I clearly see many hard edges.