http://www.digitech.com/Download_Registration/Software_registration.php you have to register to get downloads from digitech. that's where you'll find the latest drivers.
A: Do I need to install the ASIO Driver only or...
B: DO I also need to install the Digitech drivers and choose them as my input/output drivers in Options Audio? (To use GNX4 as My Sound Card?
I think you're confused about ASIO drivers. When you install the software from your soundcard (in this case your digitech gnx4) you install the drivers. those drivers can include ASIO, WDM, MME and maybe others. When you go into SHS and select ASIO driver mode, you are telling SHS which drivers to use that you installed from the mfg's software.
So yes, you install the latest drivers from digitech from the site I linked to above and that will install ASIO drivers. then you tell SHS to USE those ASIO drivers by selecting ASIO driver mode in OPTIONS>AUDIO>ADVANCED>DRIVER MODE = ASIO (then close and restart SHS). this will allow you to select the Inputs and Outputs of the GNX4 in your track for recording.
D: In regard to Choosing MIDI drivers under Options/MIDI devices, Will I use the DIGITECH USB MIDI driver as the input and Digitech USB 1-2 Audio Device as the output driver (To use GNX4 as My Sound Card?)(To hear MIDI Playback from my powered speakers through the GNX4?)
the GNX4's MIDI I/O can be selected in the MIDI DEVICES, but this will not route your MIDI playback to your powered speakers. If selected on a MIDI track, it will send the MIDI to the MIDI I/O ports on the GNX4. If you don't have a keyboard connected to the GNX4 then nothing will happen.
there are 2 things to learn about here. the GNX4 has a drum machine built into it. I don't know enough about it, but I would assume that it's MIDI driven, so to use it you need the MIDI I/O selected as a device and you'd need the GNX4 set up to send/receive the drum machine MIDI data/drum sounds back and forth to SHS. This is something that will take some learning to do and you'll probably be better off learning the 2nd thing I'm going to tell you before trying to tackle the drum machine in the GNX4.
2nd thing is that you'll need to learn to use softsynths in order to get MIDI playback (with the exception of the drum machine). tutorial #8 in SHS will teach you how to use softsynths. the MIDI I/O port on the GNX4 will not playback MIDI thru your powered speakers - you have to use softsynths for that and then THAT will send MIDI playback to your GNX4/powered speakers.
there's a lot to learn, but it sounds like you're on the way. good luck