...
In effect, while it may be eminently practical to use CBC as the default and move gradually, we felt that changing the default going forward was the only effective means of raising the visibility of the issue.
Progress by Other Implementations
The decision to make this change has spurred some discussion by other projects to finally get GCM supported, which is good news. Other projects are welcome to add information here to assist with tracking.
PySAML
https://github.com/IdentityPython/pysaml2/issues/671