Can I use the DDJ-GRV6 without an internet connection?

Yes, you can use the unit without internet connection.
However, you'll need an internet connection to use some functions and to do certain things in preparation for DJing, some of which you can see in the examples below.

・Downloading and updating DJ software
・Downloading firmware updates
・Signing up for subscriptions and plans (e.g. software and streaming services)
・Streaming music