Man pages matching name astro::fits::header::gsd in section 3pm:

Astro::FITS::Header::GSD.3pm.en	debian 10 / libastro-fits-header-perl
Astro::FITS::Header::GSD.3pm.en	debian 11 / libastro-fits-header-perl
Astro::FITS::Header::GSD.3pm.en	debian 12 / libastro-fits-header-perl