My personal, entirely subjective impressions using the Gaia with the Chord Qutest: (I now have my Terminator Plus, I will post some thoughts about that combo later- the TPlus is still new and burning in):
The Gaia transforms the Qutest. I had to keep checking myself since the Gaia arrived to make sure I was hearing the magnitude of change I believed I was hearing. Yep, it is real. I could see the Qutest+Gaia combo as an "end game" combo for the vast majority of audiophiles.
If I had been listening to the Qutest just by itself for a long time, and then I was blind-folded and someone connected the Gaia to the Qutest, but told me they had inserted a Dave*, I would believe them and immediately start figuring out how long it would take me to save up for the Dave because hearing the improvements, it would be difficult to go without. Again, though, that is just me.
*I have not heard the Dave in my system and thus I am not saying a Qutest+Gaia would equal a Dave. I am just trying to express the magnitude of the change. After my years of buying and trying different DAC's in my system, I view the Gaia improvements to comfortably exceed one, and possibly equal even two DAC model jumps within a manufacturer's DAC line.
Another data point: I loved my Berkelely Alpha USB D/D converter and it improved every DAC to which it was connected (most recently a Yggy Analog 2). The sonic improvements brought by Gaia exceed the sonic improvements from the Berkeley by a rather large margin.
I'd love to put a Gaia up against a PhoenixUSB or Chord M Scaler just to see how it compares (yes, I appreciate these are differing devices and it would be a somewhat flawed comparison due to the different connection types). If you simply view all of these devices as "sonic improvement" devices at differing price points, which at the end of the day is what they are (thinking of the Gaia as merely a tool to convert to a different digital type is really missing the big point of the Gaia), the Gaia might prove to be a cheaper alternative for a given amount of sonic improvement.
What are the primary sonic improvements? Dramatic (on an audiophile rating scale- i.e. not to my wife's rating scale) improvements to holography/sound staging and transparency (hearing even more musical information, details, the impression of the recording venue, etc). The sound stage is huge and there is so much air between the performers. I really value those aspects. If those sonic attributes are not what floats your boat, then move along, nothing to see here.
Finally, as a Terminator Plus owner, the ability to take advantage of the TPlus' clock output to the upstream Gaia, is more icing on the Gaia cake.
