Will GDT have Steam Controller Support?

Title says it all

How does one play this game with a controller?

There are ways you could add controller support : there are only limited places on the screen you can interact with : each char, the “Fill position” button, the background, the finish button, … Plus the hardware lab, the R&D lab …
With directionnal pad you navigate between each of these places, a right trigger and a left trigger for the labs, and voilà ^^

Strictly speaking, since the Steam Controller will emulate a Keyboard and Mouse for games which don’t explicitly support it, then yes, there will be support. However I doubt GDT will be modified to explicitly support the Steam Controller, as by the time the Controller is released to the general public, GHG will most likely be working on their next project.