I'm not sure what you're referring to.

Steam itself is only available as a 32-bit binary, if I remember correctly.

checks

Yeah, on my system, looks like a 32-bit binary.

If Steam runs a game, which can be either 32-bit or 64-bit, I believe you need to have libraries for the corresponding architecture for stuff that isn't in the Steam Ubuntu-based collection of libraries, the stuff in ~/.steam/steam/ubuntu12*. If you're running a 64-bit binary, you need 64-bit libraries, and for a 32-bit binary, you need 32-bit libraries.

I have GPU libraries for both architectures installed on my Debian system, like libdrm-amdgpu1:amd64 and libdrm-amdgpu1:i386, with multiarch.