systemd - amark/gun GitHub Wiki
@BIOS mentioned this for his RasPi setup:
(venv) pi@chia1:~/gundb/gun $ cat /etc/systemd/system/gun.service
[Unit]
Description=GunDB Relay Peer
[Service]
User=pi
Group=pi
WorkingDirectory=/home/pi/gundb/gun
ExecStart=/usr/bin/npm start
[Install]
WantedBy=multi-user.target