stages of compilation gcc is a collection of compilers, based on the file extension it compiles a source file. But compilers are only supposed to convert a preprocessed code into assembly code. And after that assembling and linking is done and an executable is generated.
But several definitions says that compilers are the ones who convert source code into executable code..
i am confused. what actually a compiler is? what is gcc? 
Can anyone please give me a clear definition? And i have seen in man pages of gcc mentioning the term "compiler proper" what exactly is this?
I'm not sure there is a well accepted definition of compiler. To some extent this is an informal concept, so you can't expect a mathematical definition like that of a derivative.

Judging from GCC documentation, the compiler proper converts (preprocessed) source code into object code. Several object codes are then linked together to an executable by the linker, but that doesn't form part of the compiler proper. The C/C++ preprocessor also doesn't form part of the compiler proper.

The compiler proper is further composed of two parts: a front end and a back end. The front end converts source code into intermediate code, and is language specific. The back end converts intermediate code into machine code, and is machine specific. GCC comes with several front ends and back ends, and this is why it is called a compiler collection.