When comparing MonoGame vs Gosu, the Slant community recommends MonoGame for most people. In the question“What are the best 2D game engines?” MonoGame is ranked 9th while Gosu is ranked 50th. The most important reason people chose MonoGame is:
Support for iOS, Android, Mac OS X, Linux, Windows (both OpenGL and DirectX), Windows 8 Store, Windows Phone 8, PlayStation Mobile, PlayStation 4, Xbox One, Nintendo Switch, and the OUYA console with even more platforms on the way.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Cross-platform
Support for iOS, Android, Mac OS X, Linux, Windows (both OpenGL and DirectX), Windows 8 Store, Windows Phone 8, PlayStation Mobile, PlayStation 4, Xbox One, Nintendo Switch, and the OUYA console with even more platforms on the way.
Pro Open source
All the code is available to you ensuring you'll have the ability to make changes when you need to or even port to whole new platforms.
Pro Well-known and documented API
The framework implements the XNA 4 API, so games made in XNA can be ported to other platforms using this. This was the same API used by the Xbox Live Indie Games platform so there's lots of documentation online for it.
Pro Managed code
By leveraging C# and other .NET languages on Microsoft and Mono platforms you can write modern, fast, and reliable game code.
Pro Good community
The community MonoGame has to offer is helpful and mature.
Pro Performance on desktop
The performance on desktop platforms matches that of C++, but you still get all the pleasant features that C# has to offer.
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
Cons
Con Slow rate of updates
Versions 3.9 is overdue by a year, and version 4.0 is set to release in 2040.
Con Non-Windows tools are a bit funky
Monogame support for Xamarin Studio or Monodevelop is a bit shaky especially for library references. Only good non-Windows IDE compatible with MonoGame is Rider and that costs money & isn't open-source.
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.)