Traditionally, the term gremolata is used in Italian cooking to describe a topping consisting of a fine mince of parsley, garlic, and lemon zest. However, chefs in professional kitchens will use the term more broadly to simply describe an herby topping which could be dry or mounted in oil used to brighten a dish.
To make a traditional gremolata, parsley is submerged, picked, dried and the. Chopped to a mince. Garlic is then finy minced and spread out on parchment to dry. Then lemon is zested and them finely minced and also allowed to dry. Once fully dried, the components are mixed together and used as a finishing comonent to top a dish and provide a hit of freshness and brightness. Classically, osso bucc, braised veal shank, is served over polenta and is topped with the braising liquid as well as gremolata.
As previously stated, gremolata is generally used more broadly to refer to an herby topping. Chopped parsley, dill, and orange zest mounted in olive oil used to finish a scallop set Is an example. Parsley, chive, oregano, lavender topping lemony potatoes is another example.
You can draw a line to the term "persillade" in French cooking to illustrate the same concept. Persillade meaning "finished with parsley" will typically mean an herby topping featuring parsley used to finish dishes in French kitchens.