Tero Koskinen has released OpenBSD/amd64 GCC/GNAT packages:
I recently got an amd64(x86_64) system and put OpenBSD there.
There was no native GNAT for OpenBSD/amd64, so I had to
create one by cross-compiling the compiler on i386.
According to Tero, the packages should work with OpenBSD 4.6-current and 4.7-beta.
The packages can be found here.