I know that 25c vs 28c won't make much [if any] difference in speed so long as they're aired to the right pressures but if it helps with gravel on the path that would be great .

I can ride over the gravel now without feeling unsteady but it does worry me a bit that I'll get a flat which I suppose would be a worry with any higher pressure tire.

I know my Trek just rolls right over it with those cushy 35c's without a concern . On the road bike I've found myself taking the lane when needed when the shoulder is terrible with gravel .