There is no communication there - only transmit a set of data and receive a set of data. DSC is a calling service with many aspects - you can simply call another boat (or group of boats) privately and the service automatically switches their radio to the channel you want to talk on, or you can request a position report from them, or you can send a position report, or (most importantly) you can hit the red button that tells people you are in trouble and where you can be found.

GPS is not required for the "telephone call" part or the emergency call part of DSC. But it is required for any functions that rely on position data - and an emergency call is not as useful without the position data.

When you want to call a vessel you see on AIS, you use DSC - clear as mud!

Mark