It is extremely good. There are two models actually, Gscart(discontinued)and Gscart Lite which is the one I currently have. It has 8 SCART inputs, 2 SCART outputs with no quality loss according to retrorgb and others who have tested it.
It even automatically switches inputs when you turn the console on so no need to have a remote or push buttons to switch to a different socket. If I remember correctly, Superg is talking about releasing a manual override board should you need it (you won't unless you are comparing console revisions or using sync on green over scart which the lite doesn't support atm).
Additionally it even has a built in sync stripper function that can be toggled on or off if your device needs CSYNC and your cable is using Luma or Composite Video as sync. Just make sure if you get CSYNC scart cables for your systems that they are properly attenuated to 75 ohms. Retro console accessories and retro console cables both properly attenuate their cables so you should be good to go if you buy from them. Best case scenario if you don't have it attenuated right and you plug it into the Gscart it will just drop sync, worst case is that it will fry the chip inside the scart switcher.
The regular Gscart is pretty much the same but one of the outputs is a Dsub style output. This allowed people to grab a cheap HD15 to 4 BNC adapter from monoprice instead of making a custom SCART to BNC. I think this model also has a low pass filter, but not sure.I think you can do sync on green on port 8 as well as its the default port that is on when it doesn't detect sync. In any case this model has been discontinued.
http://retrorgb.com/gscartsw.html