I currently downloaded the source code for gameplay game engine .
There is a .doxy file along with the code.
I am not able to open it or use it.
I tried even on Internet.
It says .doxy is a documentation system.
Could anyone help me open it and use it please..
That is a Doxygen configuration file. You will need to download and install Doxygen for your OS and compile the help files. Once installed, you just have to add Doxygen to your Path system environment variable (in Windows) and run "doxygen" from the command line where the doxy file is located. Depending on the configuraiton options, it may create HTML, CHM, or RTF documents. It can also take a while to compile the API documents if there's a lot of source code involved.
Keep in mind this will only generate an API document (similar to JavaDocs). It's only as good as the comments they put in the source code. It's in no way the same quality as a Wiki page or dedicated online help. They have a gameplay-docs folder with some pdf files in there. You would probably be better off reading those first.