Here's what's likely happening:
* You might be thinking of "Godzilla vs. Mechagodzilla II" (1993). This film features a robotic Godzilla known as "Mechagodzilla" (not Mechagigan).
* You might be thinking of "Godzilla vs. King Ghidorah" (1991). This film features a smaller robotic kaiju called "Mecha-King Ghidorah".
It's important to remember that there are many Godzilla movies with similar-sounding titles. So, double-check your memory or any information you have on the movie.