Can't build qtwebengine for ios

I am trying to build qt from source for ios. I am able to build all modules except webengine.

The configure error is:

CMake Error at qtwebengine/src/CMakeLists.txt:126 (message):
  No gn found for cross-compilation

When looking at the code I find that cmake searches for a gn package with version 6.5.3 (like the qt version number). How m I supposed to find this gn version ? Shouldn't it be automatically installed ?

Thank you for your help.