Apart from the sections that are explicitly stated as being 'off road' or on disused rail alignments is the vision that this would be a Victorian-style tram line with the rails 'in the middle of the road'?
I am not clear that this would offer much by way of faster journeys than the bus although there may be specific benefits in terms of slightly better town/city centre penetration through otherwise pedestrian areas or avoiding one-way systems.
Besides slightly better environmental credentials than buses (which, one assumes, might become decarbonised by various means in due course anyway) what are the principal benefits to customers?
A single trunk route with no branches suggests that many public transport users would have to change to bus to reach other destinations. Is a 'feeder' re-configuration of the bus network envisaged?
Although I have strong family connections with Cheltenham and Gloucester I am not really familiar with the area these days but it definitely sounds like an interesting idea that I would like to understand better.
Certainly within the urban areas it would have to be "rails in the road", a bit like the bits of Metrolink in central Manchester. Outside of urban areas I've mainly followed existing roads, because that's the easiest method of drawing lines on a map! In the incredibly unlikely event that someone actually decided to progress with a scheme like this, I have no doubt that more suitable alignments could be found that are cheaper/easier to build/better serve the area.
In terms of journey time benefits, there probably wouldn't be much of a benefit against buses, unless a more segregated alignment can be found. As for customer benefits, I imagine higher capacity and more reliable journey times might occur? Besides, who doesn't like a tram?

(In other words, more development work is needed!)
My route is a single line simply because I've joined together a few bus routes that happen to line up: between Bishops Cleeve and Cheltenham, Cheltenham and Gloucester, plus three different park and ride sites. Feeder buses should be perfectly feasible, although I imagine many passengers would prefer to retain direct buses to the town centres. Obviously someone with access to bus passenger statistics could change the route/add branches etc based on demand (or say "what idiot thought of this? There's no way it'll be viable!"

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Hopefully this is helpful. But as I said I don't think this will ever actually happen!