It depends on your taste. Gantz has some intense action and a complex story, but it can be quite violent and dark for some. If you like that kind of stuff, it's great.
Mangadex might have uncensored manga. But it's important to note that the presence of uncensored content can change over time and is subject to various factors such as publisher restrictions and site regulations.
It varies. Sometimes you can find uncensored manga on Mangadex, but it's not a guarantee for all series. It really depends on the uploaders and the availability of uncensored versions.
Downloading manga from Mangadex is not a legal or ethical thing to do. You should support the creators by purchasing or accessing manga through legal means.