Hi Otto,
For your kind information, the current behavior of Gantt is the 0 days lag task will start from the next day to the predecessor task, which is same as in the Microsoft project. Please refer the below screen shot from MS Project for your reference.
If we want to start a task from the day, the predecessor task gets end, we must mention the -1 day to achieve.
As in the above screen shot, Task2 is 0day lag from Task1, and Task3 is -1 day lag from Task2 as specified in the Predecessors field.
Please let us know if you need more information on this.
Regards,
Mahalakshmi K.