Untrue. G Gundam's influence cannot be overstated. There are many, many manga and video games that reuse G Gundam's plot and characters nearly verbatim. It was also most Japanese viewers' first exposure to Yasuhiro Imagawa, who is one of the defining mecha genre directors.
To put it a different way, the ending of the parody baking manga Yakitate!! Japan is only comprehensible if you're familiar with the plot of G Gundam and recognize that it's being parodied. That's how famous the G Gundam plot is-- you can assume everyone reading a manga in Japan recognizes it.