When comparing Polycode vs Gosu, the Slant community recommends Polycode for most people. In the question“What are the best 2D game engines?” Polycode is ranked 32nd while Gosu is ranked 50th. The most important reason people chose Polycode is:
Polycode uses Lua and C++ to create native applications for Windows, Mac, and Linux. Mobile platforms are planned for the future.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Can create portable native applications
Polycode uses Lua and C++ to create native applications for Windows, Mac, and Linux. Mobile platforms are planned for the future.
Pro Free and open source
Polycode is free and open source (released under the MIT license). The source code is freely available on GitHub.
Pro IDE for 3D editing
Recently, the editor and its integrated tools have been released in compiled form. Features coding and scene editing in one application.
Pro C++/Lua
Polycode is distributed in a C++ form as well as with IDE. And there's an additional LUA-based scripting system.
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 Abandoned
The project does not seem to be maintained anymore: there has been no new release nor commit since mid-2015.
Con No mobile support
Doesn't have support for Android or iOS.
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.)
