Now that X and Y have Dark Pulse as a TM again. Is it possible to produce a Hydreigon with both moves? I know this was not possible in the previous generation. would transferring a Garchomp forward that knows Earth Power and produce a Deino that knows the move can I teach it the DP TM?
Yes it would happen, like you said just have a male Garchomp with Earth Power and a female Deino,Zwelios, or Hydriegon and your offspring will have Earth Power.
Hope I've Helped