Hello @girardcarillo,
Unfortunately I don’t think ROOT supports the full gdml schema and things such as loops are not implemented. There are some older posts suggesting this is the case, and I don’t think this has been implemented since: GDML <loop> tag - #2 by agheata, GDML loop tag v5.22.00
Taking a glance into the GDML parser source code I cannot see anything related to gdml loops: ROOT: geom/gdml/src/TGDMLParse.cxx Source File (around line 390). Perhaps @agheata can confirm this.
Personally what I do when I want to work with a very complex geometry is to use gdml but not write it directy and instead use a tool such as python to write the gdml and leave all the complexity there. This (GitHub - SciProgCentre/gdml.kt: GDML bindings for kotlin-multiplatform) is a tool to do this, you might find it useful, but its not actively maintained and you might face some issues.