I have GD on steam with the crucible. Could I just get the Ashes of Malmouth gog dlc and play it on top? I understand it will be on galaxy and not the steam network. Just wondering if I need to buy the full set again to play the new dlc on gog.

No, you need to buy the base game on GOG to then be able to buy and play any of its DLCs available here. GOG will not let you checkout Ashes of Malmouth if it doesn't find GD in your library.

They are separate platforms so you can' t put a GOG AoM on top of a Steam GD base game.