This is a follow up to my above comments, which may allow you to address your issue within Revit, prior to generating the gbxml file. I just looked at a simple two space test model in Revit and found that the order that spaces are placed in Revit, will impact whether the internal floor surface is coded as "T" top of the lower upper space, or "B" bottom of the upper lower space. In the first image, space 5 is the upper space, and it was placed first. In this case, it "owns" the floor surface and it is named as the "B" (bottom) of space 5; The surface name is: "B-5-6-I-F-10"

In the second image, space 8 is the upper space, and it was placed second. In this case, space 7 "owns" the floor surface (presumably since it was placed first) and the surface is named as the "T" (top) of space 7; The surface name is: "T-7-8-I-F-12"

This is As a follow up to my above comments, which may allow you to address your issue within Revit, prior to generating the gbxml file. I just looked at a simple two space test model in Revit and found that the order that spaces are placed in Revit, will impact whether the internal floor surface is coded as "T" top of the upper space, or "B" bottom of the lower space. In the first image, space 5 is the upper space, and it was placed first. In this case, it "owns" the floor surface and it is named as the "B" (bottom) of space 5; The surface name is: "B-5-6-I-F-10"

In the second image, space 8 is the upper space, and it was placed second. In this case, space 7 "owns" the floor surface (presumably since it was placed first) and the surface is named as the "T" (top) of space 7; The surface name is: "T-7-8-I-F-12"