Hi Philipp, I think what should work is if you already pro-actively delete the 3 "Window Actions" (ir.act_window) that refer to this type of Gantt view, before deinstalling. This assuming that you get the error UPON deinstallation.
If deinstallation succeeded but you still get this error, you should try to hard-refresh the browser screen and restart your Odoo instance, to make sure all caches are cleared and nothing refers to BryntumGantt anymore. If that also doesnt work, there might still be a window action left that refers to the BryntumGantt type, so you should remove that too.
Finally, if all else fails you can also leverage OCA's "database_cleanup" module to purge the module and its records, but that also comes with some risks.