but maybe it should advertise that it is pure Perl

Personally, as you're the first to populate that namspace, I would say simply leave it at Astro::FITS::Parser (or maybe Astro::FITS::Parse), and not put something like PP in the name.  If anyone later comes up with a C implementation, they can always add ::XS to the name of their variant.  I don't think there's any clear CPAN convention whether to indicate the type of module with PP or XS (there are both variants to be found).  Just mentioning in the docs that it is pure Perl should be sufficient, IMHO.

As for whether to use Astro::FITS::GSC vs. Astro::GSC::FITS, I think it depends on the semantic relation of GSC and FITS.  If FITS is just one GSC format among others, I would suggest putting the GSC first, while if FITS may also store other data, using FITS::GSC might be more appropriate  (I'm no expert in the field, so I can't tell...)