This will occur if you run out of memory while the gravity script is working. You will need to increase your swapfile size to 500MB or more.
This can be done in dietpi-config, under “Advanced options” then reboot to apply the changes.
Not sure about the linuxdash one, but the syslog error is expected, its harmless. Syslog is not installed by default on DietPi systems.
If log files are critical to your systems operation, you can use dietpi-software to set your Log system to "full"