Yes, but XC do provide a fast hourly link from Gloucester to Birmingham, so that direction is already catered for. While it goes to Derby and Nottingham and not Manchester or Newcastle, direct links to everywhere aren't possible.
Is a link from Gloucester to stations beyond Bristol that important in the grand scheme of things?
Running the existing XC services to Plymouth via Gloucester is fraught with issues as the onus to avoid Gloucester to save time in the instance of late running is obvious, so calls at Gloucester would come with some inherent unreliability. Moreover, reversals are a nuisance to timetable and operate, particularly if they are at broadly similar times in each direction. Through passengers with a forward facing preference don't like reversals either.
While valid points about other operators, I doubt there is any desire to increase frequencies in the Gloucester area. The peak service to Bristol is already quite frequent, with five departures before 8am, one of those at 0700 to Penzance. There is a train back from Penzance as well
One off-peak train an hour from Gloucester to Bristol extending to Worcester every other hour is an established level of service with few expecting more, despite some discussion about it prior to 2020. Stopping at Yate and Cam & Dursley only adds about five minutes.
So a station on the avoiding line is really just an awkward status symbol for the city, borne of the desire to have longer distance links, almost like the debates people have about other places not on the XC network.
While that is all perfectly reasonable and rational from an operating a railway perspective, it could equally be used as manifesto for opening an hypothetical station on the XC line!
Cheltenham and Gloucester are roughly same size (I'd argue Gloucester is larger but let's say they're the same). Basically all current services pass through both settlements, but Gloucester has a more limited service as basically no fast XC services call. Looking at the passenger figures pre-covid it's 1.5 million for Gloucester vs 2.5 million for Cheltenham, over 60% more passengers. I would also point out that while both stations are fairly poorly served by road due to traffic issues, Gloucester is better located for the city centre and major employers such as Gloucestershire Royal Infirmary, whereas Cheltenham is poorly located for the two centre, hospital, uni etc, but is well located for the low density suburbs. So all things being equal you'd expect them to be broadly same (or Gloucester even slightly ahead given its better commuting potential, both outwards to Bristols and inwards) but there is a significant difference.
In terms of the existing Gloucester-Bristol service you note that they only stop at Yate and Cam & D but they also stop at Filton Abbey Wood, adding another couple of minutes. There are also plans to add additional stations at various locations including Charfield (MetroWest2), Stonehouse (Stroud MP has got money from Gov to look at this) and Tuffley/Quedgeley(council led??). Clearly not all these are going to go ahead but even if only one does that's another couple of minutes added to the journey (five intermediate stops before Temple Meads). The current services are likely to be held for other faster services including XC, and they are operated by slow 30 year old DMUs, with no sign that they will be replaced by something modern or faster. I don't use the service that regularly, but pre-covid rush hour trains (of which there is 1 an hour PM) were regularly standing in both directions between Bristol and Yate. And that's before even starting on Birmingham services, with only an hourly service compared to a roughly three an hour at Cheltenham (two of which don't stop anywhere on route). None of this is conducive to attracting folk from Gloucester off the M5.
Now as I've said twice already I'd personally be extremely sceptical about such a station but there are obviously issues around the service provision at Gloucester, which is clearly suppressing demand. I think everyone would agree reversing XC trains is a mistake, so the obvious alternative, for those who maybe don't fully understand the situation (especially around XCs own issues) is a new station on the mainline...