Version 1.4 of the Agunua library has been released
<https://framagit.org/bortzmeyer/agunua/>. Among the interesting
things, the command-line tool geminitrack can now turn a capsule into
a gempub file. Gempub is a format for Gemini-like ebooks
<https://codeberg.org/oppenlab/gempub>.
Lagrange 1.4, released today, can read gempub files.
So, for instance:
geminitrack --gempub --maximum-files 100 gemini://foobar.example/
will produce a gempub, foobar-example.gpub, which you can easily read
offline.