The anime did add a bunch of scenes of him teasing Giroro and even being annoyed when Giroro shows fondness for Natsumi a couple of times. It's anime-only though, and really started happening out of nowhere in the later seasons. And, obviously, they never actuall play out seriously, it was basically fanservice since a large part of the Japanese female fans seem to ship Giroro and Kururu.
Well, this is a heated debate that has been going on for years on whether or not Kururu is LGBT.
Hell, I remember in 5th or 6th grade where I found screenshots from a Chinese Keroro Gunso yaoi doujin from some DeviantART users shipping KuruGiro. Then a few years after that incident, I finally found the original doujin and then...I can't really tell more about this since it's just too cringy and stereotypical.
The though of being stranded on a primitive planet alongside other 4 people of the same species allows him to literally do whatever he wants, including flirting. It's an opportunity for Kururu to take a break from a militaristic-type culture and sabotage his own works knowing that his own Leader Keroro slacks himself. While it is quite stereotypical, it's possible that he's just a chaotic Bisexual/Pan person.