When comparing Gosu vs Adventure Game Studio, the Slant community recommends Adventure Game Studio for most people. In the question“What are the best 2D game engines?” Adventure Game Studio is ranked 23rd while Gosu is ranked 50th. The most important reason people chose Adventure Game Studio is:
Good for newbie game creators. Can be used for prototyping: on several occasions was used to make a demo/experimental version before creating a final commercial product on different engine.
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 Easy to learn tool
Good for newbie game creators. Can be used for prototyping: on several occasions was used to make a demo/experimental version before creating a final commercial product on different engine.
Pro Completely free and open source
AGS is licensed under Artistic License 2.0 and is completely free for use for creating both freeware and commercial games.
Pro Relatively well documented
Besides the manual there are multiple text and video tutorials and code samples written by community.
Pro Used for a number of high-profile commercial releases
Adventure Game Studio has been used to develop games such as "Resonance", "Blackwell" series, "Gemini Rue", "Primordia".
Pro Lots of assets available
An extensive library of game templates and script modules accumulated over years. You can construct a simple game in hours (if you know what you are doing).
Pro Friendly community
An old, big and active community which would support newcomers not only in learning basics of the engine, but can help with every aspect of game making (including art, voice acting, moral support, etc).
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 Natively supports only 2D
2D only native support, 3D could be supported with plugins though.
Con Uses dated tech
Engine is based on the old technologies, which impose number of limitations and may cause problems on latest systems (level of annoyance varies depending on your priorities).
Con Graphics renderer is a bit dated
Graphics renderer is not well optimized for high-resolution games and complex effects.
Con No visual editor for scripts
You have to actually write all scripts yourself.
Con Development is slow
Further development of the engine is currently slow, done by only few people in their free time.
Con AGS Script isn't as full-featured as other scripting languages
Its own scripting language has lower syntax capabilities compared to modern script languages.
Con Assets cover almost exclusively adventure/quest genre
The features, script functions and game templates are very biased towards adventure/quest genre. The non-adventure games were made in AGS (2D shooters, platformers, turn-based strategies), but their development usually requires to write everything from scratch.
Con Workflow is closely coupled with the editor
Workflow is very tied to the editor and custom file formats, which can cause problems for bigger, more professional projects (interfering with source control, parallel development, automated builds, etc)
