If you use find_package (Gtk2 ...), you do not need to use pkg-config at all. CMake will find the right flags for you. It also works for operating systems such as Windows, where pkg-config is missing.
HOWEVER, if you insist on using pkg-config, follow these steps:
find_package(PkgConfig)
if (PKG_CONFIG_FOUND)
pkg_check_modules(GTK "gtk+-2.0")
if (GTK_FOUND)
target_link_libraries(yourexecutable ${GTK_LIBRARIES})
add_definitions(${GTK_CFLAGS} ${GTK_CFLAGS_OTHER})
endif()
endif()
"pkg-config -cflags" CXX_FLAGS, , Gtk2 "pkg-config -libs"
EDIT: , , pkg-config , "CFLAGS". "LIBS" ( ), , .