The Cais de Gaia waterfront is lined with restaurants, most of them touristy and priced accordingly. The view is excellent — you look directly across the Douro at Porto's tiled façades — and some of the fish and seafood restaurants do justice to their ingredients, but you will pay tourist prices. The grilled fish, especially the dourada (sea bream) and robalo (sea bass), is usually good; bacalhau (salt cod) appears in countless forms. Ask what fresh fish is available rather than ordering from the menu.
For something less theatrical, move inland from the waterfront into the streets of the old town above. The area around the market and the cathedral has smaller tascas with local clientele, better value, and plainer cooking. This is also where you will find the local specialty of Gaia: a variety of confectionery pastries and sweets that the convents of the area have been producing for centuries, particularly the conventual sweets made by the Corpus Christi nuns.
The port lodges themselves — Taylor's, Graham's, and several others — have wine bars and restaurants attached that are above average for the category, though expensive.