Amazing file..... ;-) You received an openGIS GML XML File with a lot of LineStrings. Remember XML Format is a container file , only.
The containing data is described by a schema, here with your file: http://www.safe.com/xml/schemas/FMEFeatures FMEFeatures.xsd
That does not usually fit like a DTM - Triangulation with Points and Shapes!
So....
The way it works with InRoads is to get a LandXML File form you Civil 3D party.
Sure there are ways to import a GML file with Bentley Map!
Talk to the Civil 3D guys they may export a LandXML file with DTM Data!