Sorry for my English. It's not my first language.
Why doesn't Gajuvia brotp make sense to me? An outburst.
If we look at the Phantom Lord guild, it was not a place to form or flourish friendships. Therefore, I do not think it is plausible that Juvia and Gajeel formed strong bonds in that place.
Another point is in Juvia itself. Always sad and serious, with the rain chasing her, this because she had no love in her life. Nobody wanted to be friends with her (she complained about it herself). If nobody wanted to be, and nobody was, her friend, that includes Gajeel.
In this case of rain, if the connection between they was as strong as between brothers, I don't think it would rain on Juvia as it was presented to us. She would have a precious friend, she would have felt love in her life. She would be happy. It was not what we saw!
You see, in the beginning he was retarted as a bad and cold guy, distant from everyone, always alone (Juvia even says that). If he was always alone, how was he a friend / brother of Juvia !? It makes no sense.
Juvia herself was somber and kept people away (Gajeel says that). So, I can not say that the two were so close, as many say out there. At most, they were good co-workers. I just can't see them any other way, and there is a lot of evidence for that in the anime and manga.
But does she call him as Gajeel-khun !? I know. And! Look, most likely, Gajeel didn't care about her looks or her rain. It didn't bother him, not because she was, but perhaps because it was little for him. As he liked to be alone, and she was not much to talk about, at the time, he didn't bother with the girl. Or some other reason. They were two dark people, there was a compatibility of personalities. One did not piss off the other. In a way it brought them together. He didn't care about her and that pleased her, somehow. At least someone didn't judge her because she was her. But again, this did not develop in friendships, due to the characteristics of both and the environment of the guild.
However, it is undeniable that of all the possible relationships, those of the two were the closest to each other, hence the suffix 'khun', which she gave to him. He was the closest to her, the person she liked the most, among that universe she knew.
When she decided to go to Fairy Tail it was obvious, being the saint she became, or in fact, always was, she would call the one she had the most affinity with until then.
And let's be honest. When you change schools and are in a new environment, that is scary. Now imagine you, in a room where you don't know anyone and everyone there has known each other for years, it's intimidating. But suddenly, you look away and see that person, who you have never exchanged more than six words in your life, but have studied with you for years. It is obvious that, in the beginning, you join her and you are closer to her, until you fit in. That was it with Juvia and Gajeel, when they moved to the new guild.
And that was why, in that interview, she mentioned Gajeel as the person she was close. It was obvious, even though they were soon joined by Fairy Tail. Ask that same question later and I want to see the result. There will be ONE name in front of that of Gajeel.
Well, Fairy Tail has another environment, more suitable for friendships and bonds. I really think that the two became very good friends, from this moment on, not before. In my opinion it takes time to form a bond like that of Brotp, but it certainly formed, currently, I will not deny it. Therefore, Gajuvia Brotp is a phenomenon within Fairy Tail, not before. If Gajeel is Juvia's best friend and vice-versa? I believe so, if we disregard Gray. Because, of all, he is the one who knows Juvia the most, by far, and is not of today. See, whenever he is in the guild, she is with him, she go to a missions with him, they talk together. Juvia does not interact much with the other members when Gray is in the guild.
And I don't think Gajeel is overprotective with her. None of this has been shown to us. Of course he cares about her and everyone, even if he doesn't show it. And I know if I were to kick him he would be super protective with Levy, who, yes, that was shown to us. And the same goes for Gray being overprotective with Juvia.
Finally, Gajeel and Juvia are best friends, excluding Gray, their friendship was formed in Fairy Tail, before they were good co-workers and that's it. Gajeel never met Bora, she joined Phantom after being kicked by him, and I don't think he would care about that, at that time. The two had known each other, at most, a couple years before FT, but probably even less than, perhaps, a year, a year and a half.
That is just my opinion, and I reflected on it seeing what was presented to me in the anime and the manga. It was the conclusion that I drew from these sources. And for me the most underestimated friendship is between Gray and Cana, for me their bond is stronger than many there, if not all, of course excluding Juvia and Gray.
And I think it would be very interesting, for those who like these two, to write fanfics about this development that I present. I don't think I'm capable of that. But, for those who want, here is the tip and the challenge =).