SDL will treat the functionality as unsupported if the functions aren't filled in.

src/video/directfb/SDL_DirectFB_video.c

@@ -133,10 +133,6 @@ DirectFB_CreateDevice(int devindex)
     device->DestroyWindow = DirectFB_DestroyWindow;
     device->GetWindowWMInfo = DirectFB_GetWindowWMInfo;
 
-    /* Not supported by DFB, for completeness */
-    device->SetWindowGammaRamp = DirectFB_SetWindowGammaRamp;
-    device->GetWindowGammaRamp = DirectFB_GetWindowGammaRamp;
-
 #if SDL_DIRECTFB_OPENGL
     device->GL_LoadLibrary = DirectFB_GL_LoadLibrary;
     device->GL_GetProcAddress = DirectFB_GL_GetProcAddress;

src/video/directfb/SDL_DirectFB_window.c

@@ -489,20 +489,6 @@ DirectFB_GetWindowWMInfo(_THIS, SDL_Window * window,
     }
 }
 
-int
-DirectFB_SetWindowGammaRamp(_THIS, SDL_Window * window, const Uint16 * ramp)
-{
-    SDL_Unsupported();
-    return -1;
-}
-
-int
-DirectFB_GetWindowGammaRamp(_THIS, SDL_Window * window, Uint16 * ramp)
-{
-    SDL_Unsupported();
-    return -1;
-}
-
 void
 DirectFB_AdjustWindowSurface(SDL_Window * window)
 {

src/video/directfb/SDL_DirectFB_window.h

@@ -78,10 +78,6 @@ extern void DirectFB_DestroyWindow(_THIS, SDL_Window * window);
 extern SDL_bool DirectFB_GetWindowWMInfo(_THIS, SDL_Window * window,
                                          struct SDL_SysWMinfo *info);
 
-extern int DirectFB_SetWindowGammaRamp(_THIS, SDL_Window * window, const Uint16 * ramp);
-extern int DirectFB_GetWindowGammaRamp(_THIS, SDL_Window * window, Uint16 * ramp);
-
-
 extern void DirectFB_AdjustWindowSurface(SDL_Window * window);
 
 #endif /* _SDL_directfb_window_h */