you can create script DCI with the following script:

// Read default gateway from DCI
gw = GetDCIValueByName($node, ".1.3.6.1.2.1.4.21.1.7.0.0.0.0");
if (gw == null)
	return null;

// Find node which will be ping source
pingSourceNode = FindNodeObject($node, "netxms");
if (pingSourceNode == null)
	return null;

// Return ping RTT from ping source node
return AgentReadParameter(pingSourceNode, "ICMP.Ping(" . gw . ")");

This script reads default gateway address from existing DCI and then use agent parameter ICMP.Ping to ping it from another node. Name of ping source node should be set in FindNodeObject function call.

Best regards,
Victor