First of all, aspiring to anyone elses' body is destined to disappoint, but being INSPIRED is a different animal! Gwen Stefani trains like an athlete - let's not pretend those ab muscles just happened.
If you want strong, defined abs muscles there are a couple of factors. You need to train the abs from different angles and you need to vary your training so that they don't just go "oh, this again? Whatever."

That's right, your muscles have an attitude problem and given the option, they'll just laze about. Give them something new to chew on!

Also, having strong abs is one thing, but if you want to see some definition, that layer over the top is not going to budge just because you're crunching and planking like crazy. You need to focus on healthy fats from fish, nuts, avocado or coconut oil for omega 3s as
well as lean proteins. Cut the fried foods and beware of stress and lack of sleep. Both cause a rise in cortisol production, enhancing the belly fat storage. Also, if you're serious, cut the alcohol. If you can't make my Ballet Sculpt classes, take on the 2-Week Plank Challenge at home! Hold as long as you can, then collapse and start again.