Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
The guild would likely be shocked at first. They consider Natsu as one of their own, so his attack would be completely unexpected. Some members might try to talk him down, like Lucy. She has a close bond with Natsu and would probably try to reason with him.
Natsu would be heartbroken. Fairy Tail is like his family. He'd probably go on a solo journey to clear his name and prove his loyalty. Erza would be stoic but deeply hurt. She'd start an investigation within the guild, using her influence and intelligence to uncover the truth. She might also isolate herself a bit while she figures things out.
Gray would be angry and sad at the same time. He was Natsu's rival but also his friend. He might lash out at first, blaming the situation or other people. But then he would turn his anger into a determination to find out what really happened to Natsu and seek justice if there was foul play involved. And he would also be there for Lucy and the other guild members, trying to keep them strong.
Lucy would be completely devastated. She and Natsu have a very close bond. She might go into a deep depression, constantly blaming herself for not being able to save him. Gray would be angry at first, feeling like he should have been there to help Natsu. Then he'd be sad and would probably vow to avenge Natsu. Erza would be strong on the outside but inside she'd be broken. She might try to keep Fairy Tail together in Natsu's memory. Happy would be inconsolable, constantly flying around looking for Natsu and refusing to believe he's really gone.
Their friends would likely be very excited. Lucy would probably be overjoyed and offer to help with things like baby clothes shopping. Gray might act a bit gruff on the outside but secretly be happy for them. Erza would be very protective, making sure that Lisanna has everything she needs and that the guild is a safe environment for the baby. Happy, Natsu's Exceed friend, would be constantly around, making cute comments about the baby and offering his own unique brand of support.
At first, they might be in disbelief. The members of Fairy Tail see Natsu as a core part of the guild, so it would be hard for them to accept that he has betrayed them. They would probably try to talk to him and find out what's going on. Some members might be angry immediately, thinking that he has turned his back on their family. But others, like Lucy, might be more hesitant to believe it and would look for a way to resolve the situation without conflict.
Yes, in a lot of Fairy Tail fanfiction, Natsu can be overpowered. Writers often like to amp up his powers for more exciting storylines. For example, they might give him new fire - based abilities or make his existing ones even stronger to defeat more powerful foes.
In fanfiction, Natsu's age can vary greatly. Some authors might keep him at his canonical age from the original Fairy Tail series, which is around 17 - 18 years old at the start. However, in alternate universe fanfictions, he could be older or younger depending on the plot. For example, in a time - travel based fanfic, he could be much older if the story takes place far into the future or much younger if it goes back to his childhood days.