you can execute your gulp command via pm2 by defining it in the process file at your project root.

ecosystem.json
{
    "apps": [
        {
            "name": ,
            "script": "gulp"
        }
    ]
}
Then, from terminal, run $ pm2 start ecosystem.json. 
You can also define watch options at that process file. 
for more details copy this link
https://pm2.keymetrics.io/docs/usage/watch-and-restart/#auto-restart-apps-on-file-change