I have setup Vuetify on my Vue webpack application.
My project is setup with vue init webpack my-project running Vue 2.5.2>
When adding vuetify (2.x) to gridsome, I had the same problem and had to add the vuetify import object to appOptions in main.js:
appOptions.vuetify = new Vuetify({})
as described in:
https://github.com/gridsome/gridsome.org/blob/master/docs/assets-css.md
and
https://github.com/gridsome/gridsome/issues/603#issuecomment-567056678