Can I install Gods and Kings instrument from DVD if I buy base game as a download from Steam. Or should I buy both packs as downloads or both on discs? And can I install Gods and Kings over a Game of the Year edition?
Both the base game and the expansion are Steam-only game. Buying the DVD is identical to buying it digitally in that case, so there should be no difference either way.
And yes, as far as I know, you can install it over the Game of the Year edition.