As near as I remember, the gcc version comes if you install the AS CPAN module.
As I mentioned, I am at v 5.24.
Added: I guess for v5.26 or v5.28, you perhaps would have to fork a new version like I did to add Tk in addition to Tkx in 5.26. Then add CPAN to the public reference project (in addition to Tk) if it is not already there?

I didn't try that, but I ran your suggested command in my command window, (eg cpan -i Math::LongDouble). Here are results in a readmore tag.
This is "verbatim", except that I edited out my user id on this machine.
Cheers, Marshall

Another PS: I looked at my custom AS build of 5.26 with Tk and I could add Math::LongDouble from https://github.com/sisyphus/math-longdouble to that fork, if I wanted to it that way. I do suspect that it would be possible manually add just this one module like I did for v5.24 on my machine. In the current world where computing power, memory and storage are getting to the "don't worry about it" stage, build strategies that would have been unthinkable before are now not only thinkable, but plausible and workable.

  WARNING: The sqrt and ** (pow) overloading is buggy
           on ActivePerl. (See the README file.)
[download]


Ooops ... you shouldn't be getting that warning. (Now fixed on github.)

I used the wizard to install perl-5.28.1 from the AS msi. I'm still having trouble getting anywhere - maybe there's something in the registry, from a previous installation that's causing problems.
I've pretty much lost interest in persevering with it.

Having added my gcc-4.7.0 compiler and dmake to my path, here's what I get when I try a manual build of Math::LongDouble:

C:\sisyphusion\Math-LongDouble-0.22>perl Makefile.PL

  WARNING: The sqrt and ** (pow) overloading is buggy
           on ActivePerl. (See the README file.)

Checking if your kit is complete...
Looks good
... Detected uninstalled Perl.  Trying to continue.
Have \_64\active~1.28\lib
Want \temp\activestate--------please-run-the-install-script--------\li
+b
Warning (mostly harmless): No library found for -lm
Warning (mostly harmless): No library found for -lmoldname
Warning (mostly harmless): No library found for -lkernel32
Warning (mostly harmless): No library found for -luser32
Warning (mostly harmless): No library found for -lgdi32
Warning (mostly harmless): No library found for -lwinspool
Warning (mostly harmless): No library found for -lcomdlg32
Warning (mostly harmless): No library found for -ladvapi32
Warning (mostly harmless): No library found for -lshell32
Warning (mostly harmless): No library found for -lole32
Warning (mostly harmless): No library found for -loleaut32
Warning (mostly harmless): No library found for -lnetapi32
Warning (mostly harmless): No library found for -luuid
Warning (mostly harmless): No library found for -lws2_32
Warning (mostly harmless): No library found for -lmpr
Warning (mostly harmless): No library found for -lwinmm
Warning (mostly harmless): No library found for -lversion
Warning (mostly harmless): No library found for -lodbc32
Warning (mostly harmless): No library found for -lodbccp32
Warning (mostly harmless): No library found for -lcomctl32
Failed to opendir 'C:\TEMP\ActiveState--------please-run-the-install-s
+cript--------\lib\CORE' to find header files:
 No such file or directory at C:/_64/ActiveState-5.28/site/lib/ExtUtil
+s/MM_Any.pm line 3049.
[download]

I don't understand any of that.
I don't know what or where the "uninstalled Perl" is.
The directory \_64\active~1.28\lib does not exist AFAICT.
I don't understand the direction to "please-run-the-install-script".

Also, in their perl/site/lib/ActivePerl/Config.pm, there's a sub _warn that begins:

sub _warn {
    my($msg) = @_;
    unless (-t STDOUT) {
    print "\n$msg\n";
    return;
    }
    require Win32::Console;
    .....
[download]

If AS is going to conditionally call Win32::Console, it's probably a good idea that they include Win32::Console in their distribution. (You can pass that suggestion on to them if you like.)
My first attempts at using the cpan utility resulted simply in the announcement that Win32::Console could not be found, and should be installed.
I speculate that _warn() had been called because dmake could not be found in my path ... but that useful piece of information never made it to the screen because -t STDOUT was "True" && Win32::Console was nowhere to be found.

Cheers,
Rob

I used the wizard to install perl-5.28.1 from the AS msi. I'm still having trouble getting anywhere - maybe there's something in the registry, from a previous installation that's causing problems.

That might very well be. Prior to some release number which I forget, you had to run the Windows uninstaller first before running a new version's .msi file.