hi
I have the same problem, Plus Support told me the following:
If you have tried renaming the payment gateway and it revert back to its original name, then it is likely something potentially contractual with the payment gateway which is preventing the gateway from being renamed. And this is an expected behaviour which Shopify does not have control over. If you’d like to clarify this, you may reach out to the payment gateway provider to see if they can confirm that for you.