Hello,
What is the history behind the R7 370?
Was it working fine on an other board, if so which one?
Sometimes there are compatibility issues associated with UEFI, it's a specification used on all newer hardware tied to the BIOS, Video bios, any hardware with software/firmware required to get the hardware initialised upon startup.
The Gigabyte board is not following this specification (older EFI spec) so it may have trouble detecting the GFX card.