1. Gatwick Express has brand new Electrostars on order.
2. The 442s are regional trains (effectively an electric 158), the 319s are commuter trains (effectively an electric 150.)
You said the 319s are prefectly suitable for Gatwick services - 319s transferring to Gatwick Express would have been an option.
Southern already operates Electrostars (lots of them!) and does not operate 319s hence the logistics burden of your suggestion would be significant.
Many, if not all, Express services actually run on to Brighton.
The 442s are completely unsuitable for the role they are performing, the purpose built 460s having been withdrawn from Gatwick Express service, due to small doors and limited luggage space!
Finally, and rather important when it comes to discussing new rolling stock, Gatwick Express is (or at least was when it was a separate TOC) profit making!
The 31xs are all similar to 150s in style of body. The 357s are really the electric version of the 170.
There are lots of "electric versions" of the 170s - Electrostars are the electric cousins of Turbostars full stop.
The 31xs are FAR from all similar to the 150s in style of body (unless you merely mean door placement). There is a rather big difference between the PEP-based EMUs and the Mk3 based ones (which include 320-322).