cd /lib/systemd/system/
sudo nano GPIO.service

[Unit]
Description=MQTT GPIO
After=multi-user.target
 
[Service]
Type=simple
ExecStart=/usr/bin/env /home/pi/GPIO/GPIO
Restart=on-abort
 
[Install]
WantedBy=multi-user.target

sudo chmod 644 /lib/systemd/system/GPIO.service

sudo systemctl daemon-reload
sudo systemctl enable GPIO.service
sudo systemctl start GPIO.service