What’s your thoughts on this I’m thinking it’s going to gravity circ maybe
Unless I've missed something, I think you must mean 'reverse circulation' rather than 'gravity circulation'?
If so, I assume you are worried that a proportion of return flow from the cylinder is going to reverse-flow back up the ground-floor-radiators return then mix with the flow to the upper floor radiators and then back to the boiler via the upper floor radiator return?
So, if I've understood what you're saying correctly, I think you're right, there's a potential issue. If the upper and lower rads each have their own zone valve I can't see that your going to get reverse circulation in the 'Why are my radiators hot in the middle of summer?' sense but there'll be some starvation of heat to the radiators when the DHW is heating and both rad zones are supposed to be on.
Following the 3-tee rule would prevent reverse circulation if a single zone valve controlled both GF and UF radiators, which doesn't look like it's the case here. It won't do much to mitigate this 'starvation', which is present any system that doesn't have an independent pump on each zone. Even non-return valves don't fix it because it's a reduction in flow not reversal. It's just what happens when you put heating loops in parallel.
On the one hand people accept that 'when the cylinder starts heating the radiators get cooler'. On the other hand if you don't follow the 3-tee rule, the next guy who looks at the system will point out the fact to the owner and allege, rightly or wrongly, that the system is below par as a result.
Or I may have misunderstood what you were saying.