GWR already have 3x c2c units on their books for the foreseeable to free up 387s for IET work (301/302/306) - the c2c units are quite restricted on what they can do due to some SDO hardware differences to the GWR sets; so they’ll only normally be found working alone on Newburys. They can theoretically turn up in the middle of 12 cars too, but haven’t needed to do that yet.
The 6x GX sets (on paper at least) are fully compatible with their younger GWR siblings, so will in theory be common use, as the GX ones are at GN at the moment. It isn’t however immediately clear what work they will be employed on, the prevailing thoughts being enabling further timetable uplift without needing more IETs - so further Bristol Parkway / Swindon workings, although 387 crew route knowledge is very limited out beyond Swindon, and not great to Swindon.