I tried deleting the yarn.lock file and re-running "yarn install". This error message I received seems to further indicate the npm registry is being hit.
error Couldn't find package "@bryntum/gantt-lib@1.0.0" required by "@bryntum/gantt@file:./src/lib/bryntum/bryntum-gantt-5.6.5.tgz" on the "npm" registry.
I even added "@bryntum/gantt-lib": "file:./src/lib/bryntum/bryntum-gantt-lib-1.0.0.tgz" to my package.json but that didn't make a difference.