honestly? I'd recommend (sorry to say this) that you buy true downward expanders rather than live with the gates in the DOD. the DOD is a compressor first, gate second.
firstly you need to keep the compressor out of the circuit as much as possible for your use (you just want to gate the noise, right?). so keep that threshold cranked up all the way and compression ratio down all the way.
then you can safely play with the gate, but I don't own those units any more (no particular reason, I get bored sometimes and change gear, but I also gradually upgrade my gear for better sonics, bigger name, less finicky setup. in this case both of the latter 2 were my reasons for upgrading from the DOD).
so I can't recommend settings other than experimenting. but if it's cutting out too soon, and you have the compressor disabled pretty much by doing what I suggested, then you should be able to turn the gate's trigger level DOWN (I forget name of the knob, might be threshold, but not to be confused with the compressor's threshold), until there is perfect noise cancelling when there is no noise in the room even when you walk around. then talk and it should open and you can hear the results. if there is a release knob on that gate then turn it up pretty high (long release time) so you don't get the gate cutting out the sound too soon. you want it to be as transparent as possible.
by the way, another (and maybe best) way to fix your floor noise issue is with high pass filters on the mics. turn them on if they have them. that will reduce low frequency pickup by the mics such as rumble in the floor. if you don't have that then you can do HPF stuff with a plugin in your daw later when mixing this (if you work that way). it will help a lot. but if it's an available option in the mics themselves, even just one of the mics, then it will help prevent the DOD's gate from opening prematurely and letting through some of that "mucky muck".
I'm not really proof reading so I hope I didn't say anything backwards here, rushing. and sorry I don't have time to look at a pic of the dod on google to see what controls it really has for the gate!
PS a downward expander is a much better gate than a simple single knob gate. look into them, some behringer compressors have them I think (might be wrong) and they are just better in every way than a basic gate, although back in the day I know the dod did a great job of gating for me when I needed it. I didn't use it for that though, I gated (and compressed) drums with it most of the time, very different.
PPS those rare times when I still gate something I use pretty high end elaborate downward expanding gates with full compressor type controls etc. harder to use for a beginner, easier to use for me because I know what I'm looking for specifically and how to use these devices. but for cheap sometimes
a microgate by alesis is also quite useful, and there are others out there if it's a must have. I think you really need HPF on the mics though and probably some better shock mounts on the mics. expensive but true if your mics have neither currently. also better mic stands can help. all of that costs $$$ though!
cheers
Don
