I'm not seeing public distribution for this library so it's a bit hard to test. The best I've found so far for Gantt charts in Retool is this - could that work for you?
It looks like Bryntum does have a UMD build which is a common blocker but that doesn't guarantee that it will work, and, If you'd want to access that library without it having a URL you might need to self-host Retool in a way that allows you to access the library from a location on your own server.
Let me know if I'm missing something though! If it's available on a content delivery network somewhere I'd also be happy to test