Diderot called it his robe de chambre, and was fond enough of it to base a whole essay on it.
The dressing gown had been going strong since at least the late 15th century under the term nightgown or, from the 17th century, morning gown. In the 18th century a nightgown/morning gown worn over waistcoat and breeches, topped off with a soft nightcap, was perfectly acceptable informal morning wear even out of doors. Men frequented coffee shops in their nightgowns, gentlemen in Bath could stroll down to the Pump Room in the morning to take the waters in them, and it was something of an artistic convention that scholars, artists and professional men had their portraits painted wearing them - e.g. Diderot himself and Rousseau. I suggest that the term dressing gown supplanted the earlier terms when this stopped being customary, and the garment became relegated to a much smaller role in a strictly domestic context.