This is actually a complex and difficult question. If we look for the answer from a textual perspective, we have to note that when Goethe finished the text of "Dungeon" in 1774, the Voice (From Above) was not in the scene. The voice and the message of salvation were added at a later date during one of Goethe's several additions to Faust I & II. So in the original Faust, the Urfaust, Gretchen did not receive salvation, or at least, she would not have received salvation then. We don't know what would have followed if Goethe had not added her salvation there, because later additions rejected the style of emotional frenzy with which he started and adopted Classical style, thus creating a very different work from the one begun.
If we look for the answer from characterization and plot perspectives, we might have an easier time sorting Goethe's reasoning out. Looking at plot, the far-reaching plot included the ultimate salvation of Faust in Part II.
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