And being prone causes disadvantage on ranged attacks, yes?If the grappling gnoll or gnolls were on either side or behind the target, potentially not. But assuming you're swarming the victim with a bunch of gnolls to hit them in melee, yes, they'd probably wind up with cover from any gnolls shooting from a distance.
Edit: Hang on, does Grappled even give Advantage to other attackers? I don't believe it does in 5th edition.
Normally to get Advantage out of grappling in 5E you need to FIRST grapple the target, THEN knock them Prone, so your allies who are also within 5' of the target can get Advantage to attack the prone target.
Am I forgetting a gnoll special ability?