Thank you for reporting the problem. Did you notice the problem yourself (daily use), or only after checking /var/log/pihole_updateGravity.log , as I indicated?
The file /etc/cron.d/pihole contains a line (job) with the following content (time may be different):
59 1 * * 7 root PATH="$PATH:/usr/local/bin/" pihole updateGravity >/var/log/pihole_updateGravity.log || cat /var/log/pihole_updateGravity.log
you should edit this line to include the required path e.g.:
59 1 * * 7 root PATH="$PATH:/usr/local/bin/:/usr/sbin/" pihole updateGravity >/var/log/pihole_updateGravity.log || cat /var/log/pihole_updateGravity.log
WARNING. pihole -up and pihole -r and pihole checkout (if core is included) will undo this change, so you need to reapply the fix after executing one of these commands.