[qcton]

App or something to see what people do on my wifi but i dont wont to use the gateway

[DramaKing]

You want Fing. The subscription is very reasonable. It will show you devices but not activity. It's like asking for a device that can allow you to listen in on phone calls. The Internet wouldn't be used for anything requiring confidentiality/privacy if it was that easy. Besides which, wiretapping is a felony under US federal law and in many other countries.