Quick DAC question

Anytime the source (OPPO) is connected to the DAC via digital connections it is the external DAC that is decoding.  By connecting the Gus through analog RCA to your amp you are listening to it's DAC decoded through the analog outs.  

through any digital output on the Oppo 105 means the Gustard is the DAC stage

You did not specify digital RCA, but singular we assume the RCA is a digital out of your Oppo vs a pair of RCA analogue out.

And yes, digital out means the next unit has to be the DAC,

I know I should run a physical ethernet cable into the gustard and just use the Oppo as a CD transport but that's going to be a last resort because of the location of my modem / router way down in the basement.

There’s an easy solution for this — just get a Wi-Fi extender or Mesh system and run an Ethernet cable from that to the Gustard.  Many people do it this way with great results and not expensive. 

@soix That's what I do.  My internet comes into the house on the main floor.  My system is in the basement.  I use an ASUS wireless router to send the signal to the basement. 

It's a good router.  I rarely have any issues with wi-fi.