Can you say of a woman that she is "dashing", meaning that she looks stupendous, graceful etc.?
A quick search of Google books shows that "dashing woman" has been used before, and FumbleFingers kindly did this NGram comparing the use of "dashing" with relation to men and women, showing that it is much more common to use dashing for men.
Etymology Online gives the origin of "dashing" as "cutting a dash", meaning acting brilliantly, from "dash" in the sense of showy appearance. Indeed, Dictionary.Com demonstrates that the idiomatic use of "dashing" as "make a striking appearance" is still common usage.
So yes, I think a woman should like to be described as dashing. Although you might use "fetching" if you think she wouldn't slap you. As this NGram demonstrates, "fetching" is a bit more common these days.
Dashing means 'energetic, spirited, showy, stylish' and can be used to describe a woman.
However, it would not describe a woman as "graceful", or "beautiful", or "gentle", but rather more of the "charming", "bright", "lively" description.