GraalVM is multiple projects and I feel there is often a bit of a mix-up around these:
GraalVM is first and foremost a JIT compiler written in Java that can be plugged into OpenJDK. Due to it being written in a higher level language than the original Hotspot compilers (written in C++) they are easier to write/maintain/experiment with. This mode of operation is used extensively by Twitter for example, because on their workloads it provides better performance than Hotspot, but the two trades blows in general. But this uses the standard javac compiler so it is basically just a slightly different JVM implementation.
Since a JIT compiler outputs machine code it can be “easily” modified to do so in an offline setting as well — this is Graal’s AOT/native compilation mode. This will take a long time compared to some other compilers (I don’t exactly know the reason for that, probably Java’s dynamic nature requiring more wide-reaching analysis?), but will have lower memory usage and faster startup speed compared to the traditional execution mode (but rarely better performance).
There is also Truffle, which turns “naive” language interpreters into efficient JIT compiled runtimes and allowing polyglot execution, which is a whole other dimension.
Caveat: I also haven't been using Native Images yet, though. So I can't comment on if it'll be dramatically different for that build target.