Seems to be a bad release (v44). Until it is fixed, the workaround is to explicitly specify previous version of the API to load instead of using current:
- google.charts.load('current', {'packages':['gantt']});
+ google.charts.load('43', {'packages':['gantt']});
Please note that this will indeed freeze your charts version and you will not get new features and bugfixes unless you will revert the changes.