you can assign to the surfaces a condition with a special question field #FUNC#(NumEntity)
QUESTION: Tag#FUNC#(NumEntity)
VALUE: 0
STATE:HIDDEN
That will be automatically filled when generate the mesh with the source surface number
https://gidsimulation.atlassian.net/wiki/spaces/GCM/pages/2410414606/Conditions+file+.cnd
note the extra state: hidden to not show to the users this special question.
Ans since GiD 15.1.5d developer the mesh has a pointer to its geometrical entity, then it is not necessary this special #FUNC#(NumEntity) question.
Can know the geometry of an element with this Tcl command
GiD_Mesh get element <element_id> geometry_source
Cannot ask the geometry of a node, because a node can belong to multiple entities
but can also ask for the element’s nodes, to know the list of ‘nodes of a surface’