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If you are completely new to terrain design software, we recommend starting with the Basics of Terrain Design and familiarizing yourself with the Gaea Interface in Getting around.
Gaea champions the idea of having multiple workflows to suit the artist and the terrain. You choose how you want to work. You can even change your workflow in the middle of the project without adverse effects. The Workflows section explains the main workflows in Gaea: Layers Workflow, Graph Workflow, Sculpt Workflow
The Using Gaea section explains various concepts in detail; including the Build Manager, Color Production, Importing Data, and more.