As far as I know everything to/from South Wales is going via Gloucester, adding approximately 40 minutes to the journey time.
Looking at the map, it looks as if a reversal at Gloucester will not be necessary as trains will approach from the north (diversion along the north bank of the Severn via Chepstow) and pass through Gloucester station to reach the line via Stroud/Kemble to Swindon. Reversal only required if passing through the Severn tunnel via Gloucester.
No additional calls en route either (in fact one less as Bristol parkway is left out), so there shouldn't be masses of extra passengers joining, although I believe the s.wales services are down to hourly throughout the closure, which, however, is normal for a weekend I think.
Thanks Chris - for a nervous traveller knowing what is going to happen in advance may make all the difference.