It appears that TGeoManager does not support the quantity tag.
ROOT:
root [0] TGeoManager::Import("root-test.gdml")
Info in <TGeoManager::Import>: Reading geometry from file: root-test.gdml
Info in <TGeoManager::TGeoManager>: Geometry GDMLImport, Geometry imported from GDML created
Error: Unsupported GDML Tag Used :quantity. Please Check Geometry/Schema.
Error: Unsupported GDML Tag Used :quantity. Please Check Geometry/Schema.
Medium: G4_Al, Not Yet Defined!
Info in <TGeoManager::SetTopVolume>: Top volume is My_Vol. Master volume is My_Vol
Info in <TGeoNavigator::BuildCache>: --- Maximum geometry depth set to 100
Info in <TGeoManager::CheckGeometry>: Fixing runtime shapes...
Info in <TGeoManager::CheckGeometry>: ...Nothing to fix
Warning in <TGeoManager::CheckGeometry>: Volume "My_Vol" has no medium: assigned dummy medium and material
Info in <TGeoManager::CloseGeometry>: Counting nodes...
Info in <TGeoManager::Voxelize>: Voxelizing...
Info in <TGeoManager::CloseGeometry>: Building cache...
Info in <TGeoManager::CountLevels>: max level = 1, max placements = 0
Info in <TGeoManager::CloseGeometry>: 1 nodes/ 1 volume UID's in Geometry imported from GDML
Info in <TGeoManager::CloseGeometry>: ----------------modeler ready----------------
(TGeoManager *) 0x7ff99b999e00
root [1]
Geant4:
...
G4GDML: Reading './root-test.gdml'...
G4GDML: Reading definitions...
G4GDML: Reading materials...
G4GDML: Reading solids...
G4GDML: Reading structure...
G4GDML: Reading setup...
G4GDML: Reading './root-test.gdml' done!
Stripping off GDML names of materials, solids and volumes ...
...