Guess it's been a while since we've seen one of Maria's family members swap out words for old mecha series so people forgot. "Gekiganger" comes from Gekiganger III, a parody of 1970s super robot anime. It is a show within a show from the 90s anime Martian Successor Nadesico. At first glance you'd just think it's a fun silly little thing a few characters in the show obsess over, but as you watch Nadesico it turns out that parody series plays a huge role in the story's plot.
As for what word Maria censored herself from saying: you can use your imagination.
Roll the clip!: youtube.com/watch?v=hKulEJUwkeQ