I would be very surprised if it was just a shuttle between Bristol and Gloucester, which would just be a waste of rolling stock.
I would think it some would run to Worcester and at the other end maybe Taunton.
I'm not sure why you think that. A stopping service takes between 45 and 50 minutes, so in terms of rolling stock utilisation, reasonably efficient. I'm assuming that the services will stop at Yate and Cam & Dursley - I gather the existing service was, pre-COVID very overcrowded in the rush hour being only hourly, with passenger volumes at Yate and C&D having flatlines possibly due to overcrowding, so would be somewhat odd not to call at them.
Though obviously if more units are available, an extension to Worcester would be very welcome, as the through service on the Bristol, Gloucester, Worcester axis is definitely deficient at the moment.