Define our identities? There's hardly anyone (relatively speaking, by comparison to say the US's independence day) outside of Australia/New Zealand that even know about Galipoli.
I think you're confusing external identity (how other countries see us) with national identity (how we see ourselves). Gallipoli was hugely influential in defining how New Zealanders and Australians thought of themselves and their nation. It was our coming of age, the moment when we started to see ourselves as nations rather than colonies.
(Of course, that description is a little bit exaggerated: I'm sure reality was much fuzzier and messier. But that just proves the point that Gallipoli has become our national myth. It defines us now, even if it didn't really define us then.)
(Of course, that description is a little bit exaggerated: I'm sure reality was much fuzzier and messier. But that just proves the point that Gallipoli has become our national myth. It defines us now, even if it didn't really define us then.)