What you're describing is for TCP. On TCP you can perform a write(64kB) and see the stack send it into 1460 segments. On UDP if you write(64kB) you'll get a single 64kB packet composed of 45 fragments. Needless to say, it suffices that any of them is lost in a buffer somewhere for the whole packet never being received and all of them having to be retransmitted by the application layer.

GSO on UDP allows the application to send a large chunk of data, indicating the MTU to be applied, and lets the kernel pass it down the stack as-is, until the lowest layer that can split it (network stack, driver or hardware). In this case they will make packets, not fragments. On the wire there will really be independent datagrams with different IP IDs. In this case, if any of them is lost, the other ones are still received and the application can focus on retransmitting only the missing one(s). In terms of route lookups, it's as efficient as fragmentation (since there's a single lookup) but it will ensure that what is sent over the wire is usable all along the chain, at a much lower cost than it would be to make the application send all of them individually.