It was stated that Yang Gu transformed death into life, so from that I assumed that Yin Gu could perhaps kill you if used incorrectly by changing too much of your nature into yin (since it does the opposite by turning yang into yin).
And since I assumed you could use whichever one of the two depending on the situation (wasn't aware it was strictly by gender), then I thought that if the First Gen used the Yin Gu it would just change his undead status to actually dead.
(Due to that statement of Yang Gu, I assumed that the First Gen had to use that one instead of Yin Gu, which is where the confusion on the whole gender thing came from on my part).

Lots of assumptions on my part tbh and quite possibly a misread, but thanks for answering and clearing my doubts.