You can try taking renderer from the prototype:
import { NameColumn } from '@bryntum/gantt';
...
nameCustom.renderer = NameColumn.prototype.treeRenderer.bind(nameCustom)
Hi there,
Please always open a new thread to report a problem. I have added a comment about your issue on the ticket. Meanwhile you can use the workaround shared by our team member here: viewtopic.php?p=113857#p113857