menuentry "NetBSD" { insmod part_gpt insmod chain chainloader (hd0,gpt2)/EFI/boot/bootx64.efi }
I am not sure I followed everything, but if you've already gone that far and use GRUB instead, why not load the kernel directly from there?
menuentry "netbsd -v" { insmod ufs2 set root=(hd0,gpt2) knetbsd /netbsd -v } -elge