I can only assume it goes to Gloucester only through my personal sightings, in a way..
I've only done it once this year from Trowbridge, 150104 was on the front with SWT 158890 on the rear. Once at Bristol it was advertised that the rear two coaches are for Gloucester, in this case 158890. (Booked to detach at Bristol anyway)
Last time I did it on a Saturday it was a 150/2 again with the SWT 158 on the rear. Both times I had to dash over for the 0730 Edinburgh XC service, so didn't see it afterwards, but being at the rear, and being advertised as the part continuing to Gloucester* I can only assume it's the 158.
* = Though this may have been a one off, otherwise the 158 might do a shunt so that the 150 can continue on 2E16?