Godzilla might see the pregnant Muto as a threat. After all, Mutos are his enemies. So he could start hunting it down to prevent more Mutos from being born.
The Avengers would be shocked at first. Tony Stark would probably crack a joke about 'giant naked monsters' while quickly analyzing the threat level. Captain America would be all business, immediately assessing the situation and trying to figure out how to protect civilians. Thor would likely see the Titans as a worthy foe and be eager to engage in battle, swinging his hammer with enthusiasm. Hulk might just charge at the nearest Titan without much thought, seeing them as something to smash.
Naruto would probably be both amazed and a bit scared at first. But being Naruto, he would quickly jump into action. He might try to use his rasengan against Godzilla, thinking it could do some damage. However, he would soon realize that Godzilla is much more powerful than he initially thought and would have to come up with a different strategy, like using his speed to avoid Godzilla's attacks and find its weak points.
Shinji would probably be terrified at first. He's already hesitant and scared in his normal battles, and Godzilla is much larger and more intimidating than the Angels he usually faces. Asuka would likely be more aggressive, thinking she can take on Godzilla with her Eva. Rei, on the other hand, would be very calm and observant, trying to understand Godzilla's nature. Misato would be in a panic initially, but then quickly start strategizing on how to deal with this new threat. Gendo would see Godzilla as either a new opportunity or a major obstacle depending on his long - term plans.
Some of them might be amused. For instance, Hulk could find it funny if Godzilla was shown as being weak in the fanfiction. He might laugh and say 'That big lizard not so tough!'.
Some of them would be amused. Hawkeye might chuckle at the outlandish scenarios in the fanfic. Black Widow would be more reserved but still interested. She'd be quietly observing and thinking about the implications of the story. Captain America would try to find the moral of the story, if there is one, as he always does.
The Teen Titans would react with a combination of fear and courage. Starfire, being from another planet, might have some knowledge about the invaders which could be useful. But she'd also be worried about her new home, Earth. Cyborg would probably be excited to take on the new technology of the aliens, seeing it as a challenge to improve his own. Raven, on the other hand, would be more cautious and try to sense any dark motives behind the invasion.
Robin would probably be very hurt. He might initially go into a state of shock and disbelief. After that, he could try to find evidence to prove his innocence. For instance, he might search for surveillance footage or eyewitnesses.
Ruby would probably be both scared and fascinated. She's always been curious about new things, and Godzilla is definitely something new. But she would also be worried about the destruction it could cause. Weiss might be more cautious and initially see Godzilla as a threat to the order in Remnant. Blake would likely be on guard, observing Godzilla's behavior to see if it's truly hostile or not.