Go back a few hundred million years and look at the fauna flopping around and you'll see all sorts of thins, including an unremarkable semi-amphibious fish with legs - Tiktaalik roseae. It'd be a bit unusual but you can't recognise it as a transitional between bony fish and Tetrapods until you know that tetrapods are going to become a big thing and what features end up defining the group.
Pop back to the time of Archaeopteryx and it'll appear as one of a number of flying dinosaurs, sharing some features with its close relatives and having a few novel features. It isn't until you've seen a modern bird that you can identify those features as a patchwork of features of theropods and birds.
It might be that, in the future, otters become fully aquatic and displace the cetaceans at which point a future scientist would look back and point out the remarkable patchwork of features that otters have, and their clearly transitional nature. But until we know what life will be like in the future we cannot identify transitionals.
Saying "All Species are Transitional" is equivocation
All species are transitional. Both Gould and Dawkins are aware of that and would have no reason to contradict what they believe to be true.
It's kinda true that many species are transitional - obviously not all, because any species that goes extinct is not - but it's a broader meaning of the word 'transitional' than the one being discussed. In general usage, when we talk about transitionals without context what we're talking are transitionals specifically between distinct higher taxonomic groups.
I think it's worth noting that most transitional fossils aren't strictly speaking actually transitionals between the groups they're transitional between. We have no way of knowing whether a fossil is in the direct line of ancestry or a side branch closely related to that direct line.
The existence of the transitionals we find confirms the existence of the direct transitionals, but they are likely not to be those direct transitionals themselves.