All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If the dress of the woman is loose and covers her entire body, then she does not have to wear something over it in prayer. When performing the prayer a woman has to cover her entire body including her feet and her hair, except her face and her hands as the Prophet said: "Allaah does not accept the prayer of a pubescent woman except if she wears a veil (a head covering)." [Ahmad and Abu Daawood]
Also, Um Salamah was asked: "Can a woman pray in a garment and a veil without a skirt? She replied: "Yes, if the garment is loose covering her feet." [Al-Haakim]
On the other hand, it is not a condition that the woman's dress should be sewn in a particular way, but it is a condition that it should cover her entire body except her face and hands.
For more benefit, please refer to Fataawa 86622 and 81924.
Lastly, we draw the attention of the questioner that a Muslim man or woman should perform the prayer wearing the best clothes as he/she is standing before Allaah, The Eternal Owner of Sovereignty, and the Lord of the Worlds. Allaah says (what means): {O Children of Aadam! Take your adornment (by wearing your clean clothes), while praying……} [Quran 7:31]
So it is not appropriate for a person to stand before Allaah in a nightgown.
Allaah Knows best.