Goldfish should have true aquatic plants in their tank so they can eat them. The plants help their digestion and overall health.
Some good plants to try include Ambulia, Hygrophila polysperma, H. ruba/ rubra, Elodia (during summer, but don't buy it in winter because it falls apart), Hydrilla, common Amazon sword plant, narrow Vallis, Water Sprite (Ceratopteris thalictroides/ cornuta) and Duckweed.
The Water Sprite normally floats on the surface but can also be planted in the substrate.
Duckweed floats on the surface.
The other plants should be planted in the gravel.