Please explain what you mean by this. Which GnuPG version, which OS, which shell, what is the problem.

My guess is that you mean a value of "100a" should exhibit an error and not ignore the "a". RIght this is a small error but nothing to worry about. atoi(3) ist used here.

I am using GnuPG 2.3.4 on Fedora Linux. I am referring to --list-options=show-sig-subpackets="100"a (note the quotes). The bug is that the character after the trailing close quote is ignored, rather than being treated as an invalid option and causing an error. That is, I would expect show-sig-subpackets="100"a to be parsed as show-sig-subpackets="100",a or be an error.

For clarification, the strings I have provided are raw argv elements as would be passed to execve(), with quoting already removed.

The quotes are irrelevant because they are evaluated by the shell and don't make a difference here.

This is incorrect. The following shell script reproduces the bug:

#!/bin/sh --
/usr/bin/gpg2 --list-options 'show-sig-subpackets="100"a' /dev/null

FWIW, on Unix is common to describe options as given on the standard shell.