Augustine said:
The code you get after puchasing M&T via Steam is the same you'd get anywhere else(never used the gate, but I assume same principle applies), in other words it's largely irrelevant where it came from. You can still d/l trial and plug in the code you've got to activate it.
This is untrue. If you purchase Warband via Steam currently you receive a 5x5 key while all other outlets provide a 4x4 key. The version downloaded from Taleworlds' website can only take a 4x4 key, hence the 5x5 key converter found in the Tech Support section (Town Watch). Similarly, one cannot activate a non-Steam (4x4) Warband key on Steam and get access to Steam's achievements, etc. (You can of course add it as a non-Steam game to have access to chat, etc while playing.)
Whether the same is true for WFaS is yet to be seen.