You write the Gb in the base; no chord members must be doubled because it's a seventh chord. To spell the rest of the chord you would find common tones between the last chord and the current tones (If the soprano had G in the previous chord give the soprano G in this chord). Once you've found your common tones, you'd then find close tones (if the tenor had Eb in the last chord give him D in this one). You then give the remaining chord member to the last part.