The Gaia is primarily a reclocker, but also has buffering and optical isolation between the inputs and outputs, and can select between many different types of digital inputs and outputs to multiple types as well. When used with the Terminator Plus, it can also synchronize the digital outputs with the clock(s) generated by the DAC.
The Terminator seems to really prefer the i2s input. I initially was using a Bryston Bdp-2 (with the upgraded IAC board) connected to the Terminator over AES3 (DH Labs D-110). I also tried connecting my Roon server (NUC running ROCK with LPS) directly to the DAC over USB (AQ Diamonnd) but I preferred the connection through the streamer.
When I got the Gaia, I first connected the NUC to the Gaia using my USB cable, and the Gaia to the Terminator using the AES3 cable. This was an improvement over using the streamer, but not a huge improvement. After several days with this hookup, I got a cheap HDMI cable and tried an i2s connection to the DAC. Wow. This was pretty dramatic, and that was with a cheap cable with no burnin. It continued to improve a bit more as I waited a couple weeks to receive my Tubulus Argentus i2s cable. This cable was even a bit better but no where near the difference as switching to i2s in the first place.
When I got the Terminator Plus, I hooked it up the same way, but added two clock cables between the DAC and Gaia. Cold out of the box, it wasn't quite as good as the Terminator, but after warming up for 24 hours, I think it probably reached parity and had continues to improve.
As I said, I've only had it for a week, so if it's anything like the Terminator, I expect most continued improvement for at least another several weeks. Once it's fully burned in, I may do a little experimentation with different cables and connections.