Total Recommendations: 3
Giant Robo and Shin Mazinger were directed by Yasuhiro Imagawa. Giant Robo is based on the collective works of Mitsuteru Yokoyama and Shin Mazinger uses elements from many of Go Nagai's previous projects. Both series attempt to present a comprehensive tribute to the authors of their respective source materials through an entirely new story.
Code Geass and Gun X Sword were made by the same director, Goro Taniguchi, and some of the same staff members too. Both series involve giant mecha combat, although Code Geass starts off as more realistic in this respect. Revenge is an important theme in both stories, plus they also share a number of ideas and their characters have a few similar characteristics. Gun X Sword benefits from being self-contained and more focused.
Both shows share the same basic concepts, famous characters and giant robots. The Mazinkaiser itself is a new, different mecha and everything happens within seven episodes. Shin Mazinger presents a fully alternative story that starts from the beginning, after showing us a taste of what's to come in its first episode, which can be enjoyed on its own. Mazinkaiser is also based on Go Nagai's Mazinger Z but it is aimed at a more mature audience, assuming a certain familiarity with the original material that isn't required by Shin Mazinger.