How to fix numb hands on new gravel bike despite professional fitting and shorter stem?

It's possible the riding conditions need better gloves or softer tyres than you're running. It's also possible the frame is stiffer, but gravel vs road makes that seem unlikely.

I run my tourer off-road (though my actual touring is on road), and have found that on rough surfaces or very long rides I really need to have my bars double-taped. I usually use a fairly soft EVA tape underneath something more robust with better grip, only above the hoods, where my hands are most of the time.

You can also get gel strips to go under your bar tape at the main contact points. I'm experimenting with the gel pads from an old pair of shorts under the tape at the moment. Comfy with big hands but maybe a bit thick with small hands.

Did the fitter look at the lever positions, and how close are they to your road bike? It's possible the contact points have moved to a less suitable bit of your hand. Similarly, do you vary your hand positions a lot on the road bike but not the gravel bike?

Look at the position of your shifters. It is often overlooked, people talk about positioning the handlebar by lowering/raising the stem, shortening the stem, but you can also move the shifters on the handlebar. Make your wrists comfortably straight. Many people (and shops) place their shifters too forward/low. Try to bring them back/up a little.

The first thing I would do is to compare the actual hand positions on the different bikes. Do this by actually mounting the bike, position the hands the way you are used to, and then look very closely at the angles between the back of your hands and your forearms. Do this for both bikes, and determine how these angles change. This should give you quite a clear picture of how the two bikes differ, and something about that difference must be the cause for your numbness. Once you've determined what is causing the numbness, you should be able to fix your hand position on the gravel bike.

That said, I would also suggest considering the use of ergonomic grips. I know that it's easy to think "oh, I don't need them", I made the same mistake for far too long. But they do make long riding much more comfortable.