Enum VerticalAlignment

var body = DocumentApp.getActiveDocument().getActiveTab().asDocumentTab().getBody();

// Append table containing two cells.
var table = body.appendTable([['Top', 'Center', 'Bottom']]);

// Align the first cell's contents to the top.
table.getCell(0, 0).setVerticalAlignment(DocumentApp.VerticalAlignment.TOP);

// Align the second cell's contents to the center.
table.getCell(0, 1).setVerticalAlignment(DocumentApp.VerticalAlignment.CENTER);

// Align the third cell's contents to the bottom.
table.getCell(0, 2).setVerticalAlignment(DocumentApp.VerticalAlignment.BOTTOM);