In effect, includes the C++ standard library headers <complex> and <cmath>, which provide type-generic math macros equivalent to <tgmath.h>.
The C standard library <tgmath.h> header isn't included by <ctgmath>, because it's effectively replaced by the C++ overloads in <complex> and <cmath>. That makes the <ctgmath> header redundant. The <tgmath.h> header is deprecated in C++. The <ctgmath> header is deprecated in C++17 and removed in the draft C++20 standard.
The functionality of the C standard library header <tgmath.h> is provided by the overloads in <complex> and <cmath>.