Here, the prime factors are 2,3,5,7. We ignored 2 because we needed only odd factors.

So the odd factors are 3^4, 5^2 and 7^1. So here the total number of odd factors would be (4x3x2) = 30. However, these are only the POSITIVE factors. Why didn’t we multiply by a further 2 to get the negative factors too, to get 60? Thanks

Because the GMAT presumably uses only positive factors. And so does the GRE, except in exceptionally rare cases (which should be explicitly mentioned).

Also the number of odd factors would be 5 \times 3 \times 2 = 30, not 4 \times 3 \times 2.