If GWR take on TfW 158s, and keep the ETCS in place, they still have to train their drivers on the operation of the units in Level 0. Its not full ETCS training but its knowing the basics as the system is never fully turned off.
(I Believe its something to do with inputting the headcode and some other data into the DMI, otherwise it wont let you move(?))
It would be more feasible to remove ETCS, or isolate in a way that means they wont have to bother with it and it doesn't interfere. But that is more work to get them into service.