Which is the irreducible polynomial in GF(2^256) used in AES256-GCM? I was able to find the irreducible polynomial used in AES128-GCM: $x^8+x^4+x^3+x+1$.

But I cant' find the one for AES256, I'm searching in OpenSSL implementation of AES256 but with no luck so far.
The AES block cipher supports various key sizes of 128, 192, and 256 bits, and a single block size of 128 bits. The degree-8 polynomial $x^8+x^4+x^3+x+1$ is the one used in AES-{128,192,256} for MixColumn and SubBytes.

There is no $\operatorname{GF}(2^{256})$ in GCM because GCM only supports block size of 128 bits. The degree-128 polynomial $x^{128}+x^7+x^2+x+1$ is the one used in GCM.