Hi, please refer to the "Key mapping" section in the game page: To go up to reach a G platform you need to press G near the stairs, to go down to reach a C platform you need to press A.
The order of the middle row keys on the keyboard (A, S, D, F, G, ...) are the order of the C major scale in music (C, D, E, F, G, ...).
Hope that helps :)