Well I got a sweet deal on a gnx3. I will be the first to admit that I am a technology dumbass. I run it through a vox ad30, which I love the sound of. So I run both amp channels direct and use the gnx3 as a selection of effects pedals / drum machine. Anyone else do anything like this? Or am I just "that guy" lol
You can see if it will do a 4 cable setup:

Then you can start adding some preamps and effects before the amp as well, then use the time effects after the amp fx return, the idea is to place your amp in the chain where you'd usually have the modeler in a digital fx unit.
No FX loop on a GNX3. That setup will work if you are mainly using fairly clean tones or getting dirt from the GNX distortion pedals.
You're right, just looked it up - no fx send/return unless there is a way to do the l/r and fake it somehow.

You can try running the GNX into the fx return of the amp and use its models or just run it into the fx loop and disable the models, so purely as effects unit. It has a 2 channnel switch so it might be able to also control your amp switching channels if it is the same type.
Thanks for your response everyone. My dirt comes straight from my vox. So both of my channels on the gnx are set to direct. No amp mod or cabinet mod used. On the vox I have a foot switch input and a phone/line input. Now can I run the gnx through the foot switch input? It states under the input what type of vox foot switch to use. I just don't want to kill my amp in some crazy ass way.
According to this:
Footswitch looks like the jack where you use a 1/4 cable to connect to your amp's footswitch input. Read the GNX manual for clarification.
Do not run the output of your GNX3 into the amp footswitch jack. Also, the footswitch jack on the back of the GNX3 is for attaching a F/S not for controlling an amp.
I've had one of these for years I love it! Try using the whammy pedal with delay set to a slapback setting( short repeat) it's a very fun soloing sound and covers up how bad of a musician I am.