When comparing Source vs GameApi, the Slant community recommends Source for most people. In the question“What are the best 3D game engines?” Source is ranked 29th while GameApi is ranked 58th. The most important reason people chose Source is:
Valve's official documentation website is great for newbies. It demonstrates the pros and cons of the engine (and since the website isn't maintained by Valve, but instead the community, the pros and cons are largely unbiased). There are also a number of pages dedicated to entities used within official Valve games and also community-made mods that were turned into full-fledged games by Valve. These pages explain the ins and outs of how most source programming works. There are also guides for Valve's tools which are both included in Source SDK and in any Valve-developed game.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Great documentation
Valve's official documentation website is great for newbies. It demonstrates the pros and cons of the engine (and since the website isn't maintained by Valve, but instead the community, the pros and cons are largely unbiased). There are also a number of pages dedicated to entities used within official Valve games and also community-made mods that were turned into full-fledged games by Valve. These pages explain the ins and outs of how most source programming works. There are also guides for Valve's tools which are both included in Source SDK and in any Valve-developed game.
Pro Runs on every potato pc
Since it has precalculated lighting, this engine is great for low end PCs too.
Pro Many basic entities
You don't need to code your own door or ladder etc. You can pretty much use every entity used in Half-Life 2 yourself easily.
Pro Visual logic, no programming
Of course you can program stuff into source but for level design only, source has a really easy input/output system for your level logic (e.g. doors, trigger when player walks into room...).
Pro Has a built-in video capture and editing application
Source includes Source Filmmaker, a video capture and editing application.
Pro Easy way to export or load source models to unity and maps
Pro All combinations of the features are working
In order to enable creation of 100 different kind of games, GameApi features are fully supporting all combinations of the existing features. This allows for huge amount of flexibility. The API carefully documents all dependencies between independent plugin-like modules and allows for combining their features in c++ code.
Pro Selection of 2d and 3d algorithms and techniques that construct opengl vertex arrays
The library is based on constructing opengl vertex arrays for 2d and 3d features including changing mesh colours, attaching textures, building larger worlds from smaller meshes, selection of predefined meshes like spheres, cubes and cones, Model loading from .obj files. Font support, gradient and other 2d bitmap operations, creation of 3d objects from bitmaps, Manipulation of points, lines, faces.
Cons
Con Only for mods
Normally, you can only use the Source Engine to develop "source-mods" (as Steam calls them), however the developer wiki is correct in saying Valve have a proven track-record for finding source-mods and turning them into fully-fledged games, Black Mesa Source is a good example of this, as it began life as a source-mod available for free, however Valve turned it into a fully-fledged and paid game.
Con SDK is outdated and difficult to use
Source SDK has not been updated in ages, and has instead been "re-released" under different names, e.g "Source SDK 2013 Singleplayer".
It's honestly easier to use the version of SDK included with any Source game, namely Portal 2 or DOTA 2, since both have a variant of Source SDK that is more updated than anything you can find in the tools section of Steam.
Con In alpha
GameApi has just recently moved from pre-alpha to alpha status.