Chapter 7 takes place on an incredibly hot day. Nick describes the intensity of the heat on the train coming out of New York and within the house as well. Daisy and Jordan have most certainly applied talcum to their skin to absorb their perspiration.
During this era, talcum powder was the most common product used to alleviate body odors on armpits and feet and to reduce perspiration. Daisy and Jordan are reduced to nothing but sitting still in order to perspire less in the oppressive heat.
In America, there were a few different brands of talcum powder available, some scented and some not. In Paris, talcum powder was also attaining a certain glamour due to its perfumed odor. One can still find decorative talcum powder tins from the era. Then, like now, using talc to reduce the effect of heat and humidity can be a bit of a messy affair.