Isn't Gross and Grand the same thing?
They both can translate to "big" however "gros" typically refers to something that is big in a sense of being fat, where as"grand" implies height.
If you told a woman she has a "des grosses robes" she might be offended, but that is just speculation.
Why is it necessary to have the "Les" there when it isn't "THE women's big dresses"?
It is NOT necessary.
Why are both "de" and "des" correct?
Because 'de' would refer to all women's dresses that are big just generally, and des would refer to a specific women's large dresses. When used with context only one or the other is correct.
Why could not it be "Les robes grandes des femmes"?