Since its initial guise as the Animo 2D animation tool Game Maker has been written in Delphi, however GameMaker Blog can reveal that Game Maker 9 will be written in C# which is an object-oriented programming language developed and maintained by Microsoft.
As of Game Maker version 7 both the maker and runner sides of the program consisted of approximately 40,000 lines of code. It is likely that this has since increased with the release of version 8 which added support for alpha transparency and featured a re-written sprite and code editor.
Wikipedia had incorrectly stated that Game Maker is written in a combination of Delphi and C++. There is no public C++ runner however the runner being developed which enables Game Maker games to run on the PSP console is written in C++.