No, loops are not supported and there is no plan yet to do that, as GDML in ROOT was intended more like an exchange format than a definition language. However, many new features were contributed lately in the parser, so this feature may come as contribution as well, specially if there is an example in Geant4.
If I have an existing GDML geometry with loops and I want to load this into ROOT, should I load it into the Geant4 runtime and then export to GDML in order to write out a compatible file?