When comparing Gosu vs Nez, the Slant community recommends Gosu for most people. In the question“What are the best 2D game engines?” Gosu is ranked 50th while Nez is ranked 89th. The most important reason people chose Gosu is:
Gosu is not a game development framework, only a media library that happens to be suited to game development. (Kind of like SDL in the C world.) That means the interface is relatively small.
Specs
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Pros
Pro Lightweight
Gosu is not a game development framework, only a media library that happens to be suited to game development. (Kind of like SDL in the C world.) That means the interface is relatively small.
Pro Mature API, actively maintained and developed
Gosu has been under development since 2001. It is mature and has several toolkits built on top of it to provide additional functionality.
Pro Cross-platform, even mobile, using Ruby
Pro Active community
The community is very active, so if you ask for help you'll get it relatively quickly.
Pro Open source
Completely free, no need to pay for licenses.
Pro Cross-platform development thanks to the use of mono and xamarin
Nez is built on top of monoGame and can be used the make games for many platforms (Windows, Linux, Mac, Android or IOS).
Cons
Con Deploying Ruby apps is a mess
Games built with the Ruby to .exe "compiler" do nothing more than extract your source code and Ruby.exe to %TEMP%, then run it. The code is not really compiled at all. The process for wrapping games as Mac apps is a bit nicer, but you'll need a paid Apple Developer subscription to code sign the app, or users will see a warning/error when running your game.
The only way to really compile Ruby is to use RubyMotion, which does not work on Windows and requires a paid subscription on top of the Apple Developer one.
(This Con is not specific to Gosu. Deploying Ruby code has never been fun.)
Con Just a framework
Meaning there's no visual scene designer so you'll have to write your own.