A running gpg-agent may be controlled by signals, i.e. using the kill command to send a signal to the process.
Here is a list of supported signals:
SIGHUPquiet, verbose,
debug, debug-all, debug-level, no-grab,
pinentry-program, default-cache-ttl, max-cache-ttl,
ignore-cache-for-signing, allow-mark-trusted and
disable-scdaemon. scdaemon-program is also supported but
due to the current implementation, which calls the scdaemon only once,
it is not of much use unless you manually kill the scdaemon.
SIGTERMSIGINTSIGUSR1SIGUSR2