Question about Jabra Gn9120 Dect Gap

Is the headset compatible with the Gigaset DX800 A ?

The Jabra GN9120 DECT GAP would be "compatible" according to the specification list, but it is still the wrong model for this application. The Gigaset DX600a has a DECT base station, but this is intended for extensions and not for headsets. With this Jabra model, you could just about accept a connection, but not dial a connection via the telephone keypad.

Without modification, the DX600a cannot be usefully used with a DECT headset. A wired or Bluetooth headset would be better.

You will have to find this out for yourself. The operating instructions state whether the Gigaset 600 A supports the GAP (General Access Profile), i.e. you can also connect other control units or headsets to the base. If the Gigaset 600 A does not support this, you cannot connect the headset. Most Gigaset phones do support this, but I would check the manual and/or the technical data to be sure, as I can only speak for my Gigaset SL 400A.