You'll find the block gallery in the panel located on the left side of the canvas. To open it you can click anywhere in the panel, including the vertical handle, or press
K which will also focus the search box.
To close the block gallery, you can click the vertical handle or click anywhere in the canvas when you don't have a block selected.
In the block gallery, there's a section for each data source you created, one for Transformations, and another one for Visualizations.
To use any of the blocks in the gallery, just drag and drop them anywhere in the canvas.