Gandha closes the sequence of five Tanmatras exactly as ghrana closed the sequence of five Jnanendriyas: last, densest, and distinctive to the single element, prithvi, that requires the fullest possible accumulation of prior qualities before it can appear. By the time gandha is produced, shabda, sparsha, rupa and rasa are all already established, and earth is the only one of the five gross elements able to carry gandha at all, since it is the only element dense enough, in Samkhya's scheme, to have reached the final stage of grossening.
This is why earth carries all five sensory qualities simultaneously, audible, tactile, visible, tasteable and odorous, while akasha, at the opposite end of the sequence, carries only the first. The progression is not arbitrary ornamentation; it is Samkhya's explanation for why solid matter seems, to ordinary observation, to be the most sensorially rich of the classical elements, water second, fire third, air fourth and space, strictly speaking imperceptible except by its sound, last.
Produced from the tamasic aspect of ahamkara, gandha completes that aspect's entire contribution to the twenty-five tattvas: five tanmatras, feeding five gross elements, in a fixed and cumulative order. With gandha in place, only the five gross elements themselves, fire, water, earth, air and ether, remain to complete the enumeration Samkhya calls the twenty-five tattvas.