I sysinst before you get to the partitioning step, there is an option
that goes like:
+----------------------------------------------------------------+
x What would you like to do? x
x x
x>a: Set sizes of NetBSD partitions x
x b: Use default partition sizes x
x c: Manually define partitions x
x d: Delete everything, use different partitions (not disklabel) x
x x: Cancel x
+----------------------------------------------------------------+
Just ignore the "(not disklabel)" part of option (d) [which probably
says "not GPT" in your case], select that option and it will wipe your
current partitions, ask you what partitioning type you would like
to use instead, where you can just select GPT again, and you get back
to the same screen shown above.
Martin