Missing header files on macOS Catalina

I am having issues with header files on macOS Catalina. I get command-line tool errors like the following :

/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/resource.h:443:29: error: parameter name omitted
 int     getiopolicy_np(int, int) __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);
                             ^~~

and gcc errors

/usr/local/Cellar/gcc@8/8.3.0/lib/gcc/8/gcc/x86_64-apple-darwin18/8.3.0/include-fixed/math.h:535:15: error: storage class specified for parameter 'nextafte'
 extern double nextafter(double, double);

I dig into header files and gcc on macOS Catalina use this answer as a reference point. The answer states

Regarding header locations, I have Apples headers in /Library/Developer/CommandLineTools/:

$ sudo find /Library -name stdio.h 
/Library/Developer/CommandLineTools/usr/include/c++/v1/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Kernel.

When I run the same command, the header locations are slightly different:

/Library/Developer/CommandLineTools/usr/include/c++/v1/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/sys/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/sys/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/stdio.h
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/stdio.h

As can be seen the SDKs are now split into MACOSX10.14 and 10.15 unlinke before. Another difference is with the header files. According the the answer in reference,

if you install gcc via brew, it will add headers in /usr/local/:

$ sudo find /usr -name stdio.h 
/usr/local/Cellar/gcc/8.2.0/include/c++/8.2.0/tr1/stdio.h
/usr/local/Cellar/gcc/8.2.0/lib/gcc/8/gcc/x86_64-apple-darwin17.7.0/8.2.0/include/ssp/stdio.h
/usr/local/Cellar/gcc/8.2.0/lib/gcc/8/gcc/x86_64-apple-darwin17.7.0/8.2.0/include-fixed/stdio.h
/usr/local/Cellar/gcc/8.1.0/include/c++/8.1.0/tr1/stdio.h
/usr/local/Cellar/gcc/8.1.0/lib/gcc/8/gcc/x86_64-apple-darwin17.5.0/8.1.0/include/ssp/stdio.h
/usr/local/Cellar/gcc/8.1.0/lib/gcc/8/gcc/x86_64-apple-darwin17.5.0/8.1.0/include-fixed/stdio.h
/usr/local/Cellar/gcc/7.3.0_1/include/c++/7.3.0/tr1/stdio.h
/usr/local/Cellar/gcc/7.3.0_1/lib/gcc/7/gcc/x86_64-apple-darwin17.3.0/7.3.0/include/ssp/stdio.h
/usr/local/Cellar/gcc@7/7.3.0/include/c++/7.3.0/tr1/stdio.h
/usr/local/Cellar/gcc@7/7.3.0/lib/gcc/7/gcc/x86_64-apple-darwin17.5.0/7.3.0/include/ssp/stdio.h
/usr/local/Cellar/gcc@7/7.3.0/lib/gcc/7/gcc/x86_64-apple-darwin17.5.0/7.3.0/include-fixed/stdio.h
/usr/local/include/c++/8.2.0/tr1/stdio.h
/usr/local/lib/gcc/8/gcc/x86_64-apple-darwin17.7.0/8.2.0/include/ssp/stdio.h
/usr/local/lib/gcc/8/gcc/x86_64-apple-darwin17.7.0/8.2.0/include-fixed/stdio.h

However, when I run the above command I get:

/usr/local/Cellar/gcc@8/8.3.0/include/c++/8.3.0/tr1/stdio.h
/usr/local/Cellar/gcc@8/8.3.0/lib/gcc/8/gcc/x86_64-apple-darwin18/8.3.0/include/ssp/stdio.h
/usr/local/Cellar/gcc@8/8.3.0/lib/gcc/8/gcc/x86_64-apple-darwin18/8.3.0/include-fixed/stdio.h

Where are the other header files? What should these commands return in macOS Catalina?