I'm not an expert on Abhidhamma, but I don't think that the idea of a gandhabba is found in the abhidhamma, but is exclusive to the suttanta presentation. In the Abhidhamma presentation, patisandhi-citta arises in the embryo or fetus immediately after the death consciousness occurs. If that is true, then there isn't any time for a gandhabba to travel all the way and enter into the fetus.
In all honesty though I don't know how these details really help resolve the issue. The real essence of the question is when does patisandhi citta arise or when does the gandhabba descend into the developing embryo or fetus. Do the texts give an answer to this question?
The non-doing of any evil,
The performance of what's skillful,
The cleansing of one's own mind:
This is the Buddhas' teaching.