As I recall from the terminal, the atmosphere from the ship was like the Halo's atmosphere (which must be analogue for earth like). However, the communication protocol was unknown to 343GS, which precludes humanity and forerunner (unless there is a highly divergent human group).
I'm going with a third species (unknown), as humans surviving the first halo event seem unlikely (they would have had to be outside the known universe, or be inside one of the shield worlds (as per Trevallyn / Onyx), both of which are unlikely events for humanity post the forerunner war).
Forerunners would have had a communication protocol that 343GS could have accessed (or derived).
So I'll go with option 3 - who knows. Unless of course it was early Prophets species (pre the major split, but after FTL was derived), which may have had a language protocol unknown to 343GS, but even then you would have expected 343GS to have had access to the data surrounding that species (given they were allied with humanity against the forerunners).