Both are made by Yoshiyuki Tomino and feel like 1980s anime even though they were made in the 2000s. They both have young genius pilot protagonists joining a small group opposing a nation simply because they were at the wrong place at the right time. Both anime also contain some nonsensical plot points which progress the story forward.
Why is this not recommended yet.. no idea but here why..
Both are works of the Great Tomino, and both are very light hearted really. The atmosphere of both are similar and both have this sort of goofy nature with the MCs. Also the mecha designs are similar as well, the character design is also the same.
I would say if you're looking for more of "Happy Tomino" then try these out. Overman King Gainer has hardly any body count, but G-Reco does a little, and really G-Reco is a bit more darker,and somewhat serious then Overman King Gainer. Try them out, you won't be disappointed.  read more
Both show are classified as "tomino happy", they have aesthetic and very similar designs. both have a narrative that forces you to use your mind (at least overman in the beginning, you have to lend much attention) serious difference overman does not reach deep levels of world that has g not reco, but if you really, Very creative. g reco is better itself, but both shows are recommended more for aesthetics than anything else.
Both series have similar aesthetics courtesy of character designs by Kenichi Yoshida. And either series is mecha with elements of biopunk - particularly in Xam'd, which isn't tagged as mecha.
The key difference lies in the tone/mood. Overman King Gainer is upbeat and fast-paced, while Xam'd: Lost Memories takes a more somber approach. In either case, the setting and details take a while to be fully fleshed out.
From the socially awkward and mildly irritating protagonists to the enigmatic girl with strange hair, these shows appear kinda similar. In terms of tone, both are gently comedic, mostly relying on the interactions between teenagers and authority figures with a background of war.
This again seems a weird recommendation, but there is logic. The shows have a similar sense of gently absurd humour and a social message, as well as some character parallels which are pleasantly amusing.
Pilots fighting outside of the box in battles instead of just using beam weaponry and machine guns. Both have very original and obscure mecha designs. Both have a relatable main protagonist, a geeky gamer instead of a sporty tomboyish girl.
Wacky, nonsensical, Tomino-directed giant robot shows with great action scenes. Daitarn 3 is old and a bit ugly, but like any good Plain Jane, it more than makes up for being unattractive by having great character(s) and charm.