The Communal Award, in 1932, conceded to the demands of B.R. Ambedkar and the ilk to have separate electorates for depressed classes. But Gandhji held fast unto death to undo this. So, Poona Pact resulted in joint electorates for depressed classes with some reserved seats. So, then, why, in the Government of India Act 1935, did the government include separate electorates for depressed classes?
This seems to be the rulers' method of "divide and rule", and largely to prevent the Congress from getting too strong:
...ensuring that the Congress could never rule alone or gain enough seats to bring down the government..
This was done by over-representing the Princes, by giving every possible minority the right to separately vote for candidates belonging to their respective communities, and by making the executive theoretically, but not practically, removable by the legislature. -- from Wikipedia
It also seems that Gandhi's protests sort of died down:
The Mass Civil Disobedience Movement came to an end from 20th April, 1934 with a resolution of the Congress Working Committee. The commotion created by the epic fast of Mahatmaji undertaken in protest against the MacDonald Award granting separate electorate to the depressed classes among the Hindus slowly died down on the acceptance of the historic Poona Pact by the British Government. Under the new dispensations scheduled castes gained double the number of seats allotted to them by the British Premier Ramsay MacDonald. ...When the Mass Civil Disobedience Movement was called off by Mahatmaji, Sri V. J. Patel and Sri Subhas Chandra Bose issued a joint statement from Vienna and observed— "The latest action of Mr. Gandhi in suspending the C. D. Movement was a confession of failure". -- from The Corridors of Memory by Rabindra Nath Aditya
On another note, since then Reserved Political Positions have remained in some form or the other.
(Full disclosure- the book above is by my grandfather, but has been quoted by Bidyut Chakravarty in his book The Partition of Bengal and Assam)
One of the primary aims of the Government of India Act was to weaken the rising Indian governing class, specifically, the Congress Party. For instance, Burma was separated from India altogether, and a number of Indian provinces were subdivided for "gerrymandering purposes. The idea of having "separate but equal" electorates for the classes at the bottom was to give them a certain sop to their pride, while dividing them from other Indian classes. Other provisions of the Act were meant to cater to the Princes.
These provisions didn't always achieve their intended purposes because the Indians were actually more united, (or at least less divided) against British rule than Britain thought. But they did represent the "rearguard" actions of a dying Empire.
You need to realize that the Government of India Act was passed by Britain, not India, since India was a British colony at the time. Whatever Gandhi worked out with the other Indians has no effect on what the Brits do unless they convinced them it's for better for British interests, which they didn't.