GT versus GTS, what are the difference!? Well I'm going to tell you. ALL of them.
First, the interior. The eclipse was offered instandard black or tan interiors but the GTS was available with two more interior themes; Midnight and Sand Blast. The Midnight theme consists of the regular black interior with dark blue accents on the instrument panels, seats, door panel inserts and center console. The Sand Blast theme follows the same layout as Midnight, with the exception of tan accents instead of dark blue. Thanks to the FAQ...
The GTS also got different wheels. They're still the same dimensions however as the GT, still weigh a bunch, and bend just as easily.
Now for the tech stuff:
Engine: The GTS got 10:1 pistons as opposed to 9:1 that come in the GT. They are still cast, and even flimsier than the ones in the GT. The GTS also got slightly more aggressive cams, 248/252 as opposed to 248/248. exact specs can be found here:
The GTS also got a variable geometry intake manifold. This uses a set of butterflies to change runner length inside the manifold to promote torque. There are claims that the GTS is faster than the GT but they are so close it is negligible.
Otherwise the engines are identical. Same injectors, heads, block, crank, rods, throttle body, fuel rails and regulator, etc. Same flywheel and clutch too.
The other difference is in the transmission. The GTS got a different set of gear ratios. Same open differential in either.
The axles changed a bit over the years for both GT and GTS, most have their own part number but are entirely interchangeable. Same ABS gears and spline patterns.
There you have it. Those are the ONLY differences, the GT and GTS are otherwise IDENTICAL.