I would not mount this sidecar to your bike. The sidecar frame is way to light duty for your bike. The Spirit eagle sidecars were fine little sidecars in thier day for smaller lighter bikes but with the one inch tube frame, light duty shock and 16 inch wheel so of which ran on a 15mm axle most on a one inch they just are not up to the demands that a bike as large and top heavy as the GS is.
My wife had a newer version of this sidecar made by California sidecars, the early California sidecar the exact same body as the last of the Spirit eagles (early ones the windsheild mount is a bit different) The California sidecars version has suspension that is a poor design at best but the frame is much stronger as it is made with 1 3/4 inch tubing. We had this sidecar mounted on a Moto Guzzi Eldorado. We had 140lbs of ballist mounted under the body to make it work well.
We can supply mounts for this sidecar for many bikes that it would be a safe match up for. www.bikescreen.com
can supply windsheilds.
We can also supply mounting hardware for your GS should you go with a stronger heavier sidecar.