Im sure if you are asking you are new to programming because those two software has two different task.
I think you might be mixing up some terms. For games you want a game-engine such as SDL or unity (list on wikipedia). For graphical user interfaces (GUI) you want something like Qt, GTK, wx (list on wikipedia).
You CAN use a GUI library to make games, it depends on the game type. But if you want high performance game specific features such as realtime 3d graphics and asset management then you should definitely look for a game engine.
What is SDL and SDL2?
SDL is the abbreviation of Simple DirectMedia Layer.
Originally when refering to SDL, SDL 1.2 was meant. It is the predecessor of modern SDL 2.0 (sometimes SDL2). Nowadays, when refering to SDL, it depends on context if you really mean the old SDL 1.2 or the modern SDL 2.0.
The obsolete SDL 1.2 and the modern SDL 2.0 are a set of units which provide a free, easy and platform-independent access to features needed for developing high performance games and applications. This includes easy access to graphic, sound and input handling (keyboard, mouse, joystick) for Free Pascal and other Pascal dialects.
What is GTK ?
GTK (The GIMP Toolkit, formerly GTK + 3) is a set of software libraries, that is to say a set of functions for creating graphical interfaces. This library was originally developed for the needs of the GIMP image processing software. GTK + is now used in many projects, including GNOME, Xfce, Lxde, and ROX desktop environments.
What is the difference between SDL and GTK?
|Point of comparison||SDL||GTK|
|Difficulty||Not beginner friendly||more easier to use|
|Used for||Create games||Create interfaces|
|Tools||You need to install others libs like SDL_image and SDL_mixer etc.. But actually SDL allows you to have full control and manages all them||All you need for interface (button,loading bar, etc..) is integrated which u don’t have the full control|
|Popularity||Large community And used widely in the industry, has big organisations behind it like Valve||Very Popular And used By Big names Software like GIMP|
For those who want to get started in application programming, I will recommend GTK.
For those who want to get started in programming video games, I will recommend SDL.