In terms of ethics, it depends on how you look at it. If we consider fanfiction as a separate realm where creativity has no bounds, then it could be argued that it's okay. But from a more traditional view of respecting the source material and the characters as they were originally created, it's not ethical. The Naruto and Gaara we know from the series are not meant to be in such a relationship, and this fanfiction idea seems to be more about shock value than a legitimate exploration of the characters.
It's not really ethical. Naruto and Gaara have their own established characters and relationships in the original Naruto series. Changing them in such a radical way, especially with this kind of pregnancy plot which is out of the blue and not in line with the original narrative, goes against the integrity of the characters. Fanfiction should at least respect the basic essence of the characters, and this kind of scenario seems to cross that line.