If i may jump in the conversation to add that the rule change, simplified front wing, no blown front axle also contribute to a loss chassis wise for RB, not only a different PU.
I suspect that front tire wake under steering especially is hard to straighten out, with less outwash front wing and new front axle. This leads to less and less stable downforce at the rear. I am still wondering why / how Mercedes has made this better performing than RB / Fer.
I imagine it only takes time for Newey to tackle this. After that we even may see Gasly in better shape. He might be a driver who is performing only superb when handling and balance is perfect.