Whenever you ask for a contextual menu (using e.g. the third button on your mouse) on a source file, you will get access to a number of entries, displayed or not depending on the current context.
Menu entries include the following categories:
In addition, an entry Properties... is always visible in this
contextual menu. When you select it, a dialog pops up that allows you
to override the language used for the file, or the character set.
This can be used for instance if you want to open a file that does not belong to the current project, but where you want to benefit from the syntax highlighting that GPS knows how to.
It is not recommended to override the language for source files that
belong to the project. Instead, you should use the Project Properties
dialog and change the naming scheme if appropriate. This will ensure better
consistency between GPS and the compiler in the way they manipulate the
file.