We've used the NPM/Bower/gulp method of making custom Kendo JS packages for our webapp and ran into a problem (using 2020.3.1118). Your build system doesn't seem to like Node.js v12? You are specifying gulp 3.9.1 in src/package.json, which is no longer supported, and which breaks under Node.js v12. I had to install Node v10 again to get it to work. If you are broken under node 12+, it would be nice to have that mentioned in your documentation, for instance here: https://docs.telerik.com/kendo-ui/intro/installation/what-you-need
Will you be updating the custom build system so that it works under recent Node.js versions?