Someone else suggested this on a discord server I'm on and it seems very feasible to me that GTR could receive the GWR 387's with 379's going to GWR instead. 33x 387's plus the 6 ex C2C 387's and 23 377/5's if Southeastern decided to bring Networkers back from storage would give GTR 64 additional trains which would be enough to fully replace the 313 and 455 fleets and could also allow some of the peak extra trains that 365's used to operate to return if demand ever recovered (though unlikely), or future proof the eventual electrification of the Uckfield branch (whenever that may happen). On Southern meanwhile, the 455 withdrawal has resulted in overcrowding in a few places, so this would justify a business case for extra stock.
From a fleet perspective, moving GWR 387's over to GTR instead of 379's would give GTR a more common fleet, all owned by the same leasing company too so would result in cheaper operating costs. The only issue I see with this happening is the work required on the 379's and 387's to bring them up to GWR and GTR specifications respectively, which I wouldn't be so sure is something the DfT would particularly want spend money on, but if the GWR 387's went to Great Northern instead of Southern, 387101-129 would only require Southern vinyls to bring them upto the same specification as the metro 377's since they already have Southern interiors.
This cascade would make perfect sense to do in my eyes, but I wonder if the DfT see it that way?
It seems very odd to me that GWR would tender for stock they already have and I don't think I've seen anything like it in the past. I'm wondering if GWR have already lost their 387's (excluding the HEX ones of course) to GTR and they are tendering for their replacements (which fits the bill for the 30x 379's)? Perhaps this also could be why talk of Great Northern receiving Class 379's has gone cold recently?