Instead of looking at fingerings, start with what IS a diminished chord? It's R, b3, b5, and bb7. In G that's G, Bb, Db, and Fb (enharmonically E). Your voicing has R, bb7, and b3 so it works as the G diminished by putting in the b3 and bb7.
But as has been pointed out, voicing a chord, especially something with as much potential for clashing tones as a diminished chord, requires that you know what others are voicing at the same time, and how each note of the chord you play relates to the notes of the next chord.
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