Head material and angle adjustment. Sorry the shadow is counter-intuitive for showing the surface finish but here we have RRS with anodized finish and Gitzo with enamel finish - Gitzo will have a edge against minor scratch or dings while the RRS is stronger but will show wear more easily due to shining surface.
The major difference is that RRS leg joint is closer to the apex while the Gitzo is both lower and have more vertical offset. There is a distance between the rotating shaft to the bottom of the angle lock tab in Gitzo but RRS joint is tightly close to the apex.
Not sure about real-life performance but RRS design should be theoretically more rigid leg lock wise. Gitzo win by having a more wider top assembly but this should compare to TVC-3x of RRS.
Here the compactness plays a major rule.