Crazy how people equate population with demand and rail service. Not all places are equal, not all people are equal - in terms of travel patterns, wealthy, employment patterns/sectors and so on. A highly populated slum does not need the same outbound rail service, and nor does it have as many visitors. I am not saying Gloucuester is a slum here - but making the point that high/dense population doesn't mean demand, eg Sunderland, Bradford, Portsmouth...
One might answer that it is "crazy" how some people compare the station usage of settlement A, popn 110,000 - 120,000 which has a variety of trains to London, Cardiff, Bristol, Birmingham and Nottingham, to the station usage of settlement B, which has more or less the same population as A, more or less the same train service as A, except that, in addition, it has TWO high-quality XC services PER HOUR to many of the most important population centres of the UK, notably Sheffield, Manchester, Leeds, Tyneside, Edinburgh & Exeter/Plymouth.
Cheltenham has much higher demand, healthier economy and a more popular place to visit for business and leisure, and that should be nurtured and developed further.
Of course, Cheltenham (aka as settlement B) has "much higher demand" under the circumstances as they are today. And nobody is saying that services should not be "nurtured and developed further".
What the OP was suggesting, AIUI, is that more importantly, a little more "nurturing and developing" should be given to Gloucester first, because it's been treated shabbily in the past.
And this relatively poor treatment has little or nothing to do with the intrinsic nature of the two populations, and is more or less down to an accident of geography-history (ie the location and subsequent layout of Gloucester Central station, caused by the GWR needing to build a line from Swindon to S Wales prior to the Severn Tunnel) and BR's determination to cut costs in the 1970s which led to the closure of Gloucester Eastgate and subsequent loss of Gloucester as a stop on what became XC services.
If, by chance, the station layout at Cheltenham vs that of Gloucester had not suited BR's 1970 goals, then today it could be Cheltenham citizens who would be complaining of a second class service vis-a-vis Gloucester.
The OP's thesis is that there is significant potential for further sustainable passenger growth to/from Gloucester, because this has been depressed by the lack of stops on the XC route. That begs the question, how to tap into that? As has been suggested, you could:
1) start reversing more of the current XC services at Central. (I suggest most believe that won't happen)
2) build a new "parkway" station where XC trains could stop without reversal
2a) north of Gloucester, around Elmbridge
2b) on the triangle just east of Central - Gloucester MarkyT station
2c) to the south of the city, somewhere in the Quedgely-Lower Tuffley area, well connected to the A38-M5 roads.
I think each of the above has its merits. But, as soon as some form of suggestion 2 was mooted, others pointed out that the next move would be dropping Gloucester Central from Cheltenham-Paddington trains (in a kind of repeat of the Eastgate closure disaster). Which would, I suggest, definitely happen.