There is a blog explaining how to upgrade your GNET with another BIOS.
Check here:
http://ikotsu.blogspot.com/2015/01/taito-g-net-mod-bios.html
NOTICE that version 2011 is the latest one, not 2009.
Check here:
http://ikotsu.blogspot.com/2015/01/taito-g-net-mod-bios.html
NOTICE that version 2011 is the latest one, not 2009.