DRAGON QUEST BUILDERS 2

Gratitude is gained from happy villagers, and is used during the story to upgrade your base level on the islands. On the Isle of Awakening, however, it's used as a currency to unlock new Explorer Shores, recipes, and eventually to buy items.

There are a few things that generate a decent amount of gratitude:

Farming: this was my primary source of gratitude. Build 6-10 scarecrows (or more) and fill them with seeds. You'll want at least 1 full plot of strawberries for breeding, and the rest doesn't really matter. Try to have a varied selection though to get the tablet target for growing each type of crop. Heatroot and Sugar Cane need to be planted in clean water as well. Try to recruit 2-3 of the spinning robots from Rimey Reef as they're excellent at farming. They can plant/harvest/water in 3x3 squares. Also, the 3x3 planting only uses a single seed, so put a handful of seeds in a chest nearby and they'll plant far more crops (move the other residents away using the resident's register) - you'll need to assign a crop type at the scarecrow to get them to plant seeds though.

Liquid metal slime: you can fight and tame these in Laguna Perfuma. Once tamed and living on the Isle of Awakening, they'll run away from you. If you chase them and talk to them, they'll give you 100 gratitude. You can only do this once per day.

Cooking: what to do with all the crops you've harvested? Get the villagers to cook them! Build a kitchen with plenty of campfires, and the villagers will cook everything in the chests. Wyverns are especially good cooks.

Activities: building spas, bathrooms, bedrooms and bars also generates some gratitude.

You can build a builder's bell to gather the gratitude, but I also found that idling in the village would increase my gratitude at a steady rate. I'm not sure if there's some limit to the amount of gratitude that can be on the ground, but I'd always notice that after idling for several days and hitting the bell, I'd only gain ~100 gratitude, but I'd have also gained several thousand while idle.