Well, I've got to give you both points on that one. Until recently, I didn't even know the GX existed, and it seems to have a specific purpose, to record a single guitar, from a mono input. It does appear to have stereo out.
The UX1 and UX2 do have stereo inputs via line in, or variations on the instrument/mic inputs. Clearly the UX2 is the most versatile of the three.
One thing this illustrates is do your homework, and shop carefully.
I can see where the GX would be most useful, but indeed, it cannot accept a stereo input. On the other hand, the UX2 can accept a stereo input, and would be a suitable interface for instruments that have a stereo output. Since a keyboard would have a line level output (if present) the line ins are ideal.
While my setup is somewhat more complex, with an external mixer and internal sound card, the net effect is the same as the UX2.
You can feed line or instrument signals, in stereo or mono, to the computer. And, sorry pete, this can be done with little to no latency because of the more efficient drivers and hardware of these sound cards, aka interfaces.