The gateway address on an end device is crucial because it acts as the primary access point for the device to communicate with devices outside its local network. Essentially, the gateway directs traffic from the local network to other networks, including the internet. When an end device wants to send data to a destination that is not on its local subnet, it forwards the data to the gateway, which then knows how to route it further.
If the gateway address is not included in the device's configuration, the device will be unable to send data outside its local network. It would still be able to communicate with others within the same local subnet, but any attempt to reach resources beyond that — such as internet services or other remote networks — will fail, leading to connectivity issues.
Summary: The gateway address is vital for an end device's ability to communicate beyond its local network. Without it, the device can only communicate locally and will be unable to access external resources, resulting in limited functionality and connectivity problems.