Interesting question about it possibly being in "G". I believe The Rolling Stones version on their albums "Let It Bleed" and the live "Get Yer Ya Ya's Out" were in "G" (possibly "A" because of the string relationship). But Mick Taylor played his solo on bottleneck, which might lend itself to your question? Robert Johnson however did not play bottleneck on that song, if I remember correctly. I believe Ari is correct in stating that the original is in standard tuning.