They're meaning "earth ground". Less having one already installed, which usually means having a large "Dynaplate" installed on the hull, most people leave them [earth grounds] off.
A Dynaplate is a large piece of sintered bronze affixed to the outside of the hull, in full contact with the water, used to (1) reduce RF interference and (2) offer the best path for hull bonding, i.e., reduce/eliminate electrolysis and galvanic corrosion due to the electrical differential between dissimilar metals.
Whereas the intent is to have a "clear path to ground", if you end up with severe "noise", e.g., vertical lines or other clutter, on your GPS/sounder unit (I highly doubt you will have any ...) you could also add a clear single-run ground wire from the unit's earth ground post and straight back to the battery. From my experience with OB-powered boats, you'll be fine. Also make sure you check your GPS/sounder operation when your VHF mic is keyed to transmit.
There are other methods available to preventing/minimizing stray RF that could otherwise cause interference ... you can add ferrites to the sounder leads or twist the power/ground leads going to the unit, at least a few (2-3) twists per inch is what's usually recommended. Note this does limit the usable length of the "existing" unit's power leads.