Dear @Khushboo,
Welcome to the Wefugees Community and thank you for your question.
I am bit surprised that you were able to receive the grey passport ("Reiseausweis für Ausländer") while holding the residence permit according to section 104c. Maybe you can explain some more details about the background (or is it issued so that you can travel to Pakistan to apply for a national passport)?
If you are a Pakistani citizen, I wouldn't know why you will need to obtain a visa for your country of origin. But you could always get in contact with the Pakistani embassy in Germany to ask about your individual case if things are more complicated.
I assume you are aware that you most probably need to provide a national passport if you want to change your residence permit to one according to section §25a or §25b latest after holding the residence permit according to 104c (Chancen-Aufenthalt) for 18 months?
Looking forward to your reply and all the best,
Meike