Hi Gortee,
Here is the correct syntax:
interfaceXml["interface"][0].name = "GW-172.16.120.1";
Also note that the root XML tag (<interfaces>) is not used in the expression.
BTW, for this particular XML content, where there is a single child node <interface> of the root node <interfaces>, you can omit the array index [0] from the expression, so the following expression will also work:
interfaceXml["interface"].name = "GW-172.16.120.1";