On https://sourceware.org/gdb/current/onlinedocs/gdb/Requirements.html the MPFR URL http://www.mpfr.org should be changed to the https version: https://www.mpfr.org I can see in Debian that gdb.pdf also has the http URL. I suppose that this is related to the GDB documentation in general (I haven't seen a doc or manual component for the bug report). You may also want to check the other URLs...