Specifically, I think cgi=on is to blame.
To test :
1. Start gmnisrv with systemd
2. `sudo systemctl status gmnisrv`
* notice it says how many tasks it has: e.g. Tasks: 100 (limit: 1107)
3. Make a request to a cgi route
4. Do step two again, see how tasks has increased.
5. Do step 3 thousands of times... you'll be glad you used systemd instead of just running gmnisrv in a screen, as it will just stop working instead of crashing your server.