How to use NetBSD with GDEMU?

I have downloaded the ISO for NetBSD but when trying to use it in 
combination with the GDEMU it says it is unsupported. Would it be 
possible to provide instructions on how to use Linux/NetBSD to create a 
gdi or cdi image file for use with the GDEMU for Dreamcast?

GDEMU is an SDcard replacement for the GD drive for the Dreamcast.


Thank you,

Aaron