Noctalia is a desktop shell for window managers. Tiling and scrolling window managers (such as i3, MangoWM, Niri, etc.) are only window managers. They don’t have any kind of graphical way to launch applications, there’s no dock, bars or widgets of any kind. They only manage the windows, and you need to provide the rest by installing a number of different applications. Noctalia is a shell that provides a full desktop experience with all the bells and whistles you might expect from full desktop environments like KDE and Gnome.
The previous versions of Noctalia were based on Quickshell which was a memory hog. It used few hundred megabytes of memory per each display you had. Noctalia v5 is a complete rewrite in C++, which especially on multi-monitor systems uses only a fraction of the memory usage compared to Noctalia v4.
I switched from KDE to Niri+Noctalia a few months ago. It’s highly customizable and pleasing looking environment. There are templates for different applications which applies your chosen color theme to applications so that you have unified look across all (or most) of your applications.
could it look close? eh maybe. the visual customization of it is still kinda limited. like if you’re looking to have the same customization levels of say Plasmashell you’re out of luck. If you’re ok with the “pill” look then it’s fine but beyond that you couldn’t make it look like say DWM’s bar or what have you. But I suppose if you wanted it to look like cosmic or maybe gnome it’d be more doable since they all kinda share the same principal of “pills are a great look”
As far as if it’ll play nice with GNOME? no clue. I’ve used it with Kwin in the past as a replacement for Plasmashell and it worked fairly well.