The comments on two GameFront articles: "Buying Ashley is gift is NOT required to have a romantic relationship with her, but it does help" and "The gift isn’t completely essential!". So you should be OK as long as you're doing most other things right.
If you're worried about what gift to get, the guides say:
Even when she’s unconscious in the Citadel, visit her and offer some kind words anyway. While there, you can purchase a special item, a Tennyson Collection, at the store terminal in the hospital lobby. You may have to wait until she’s awake to purchase and gift the special item.