Center text in Table Cell - OpenXML SDK

You can control the vertical alignment using the TableCellVerticalAlignment class:

TableCellProperties tcp = new TableCellProperties();

TableCellBorders tcb = new TableCellBorders(
        new TopBorder() { Val = new EnumValue<BorderValues>(BorderValues.BasicThinLines) },
        new BottomBorder() { Val = new EnumValue<BorderValues>(BorderValues.Double) });

//set the alignment to "Center"
TableCellVerticalAlignment tcVA = new TableCellVerticalAlignment() { Val = TableVerticalAlignmentValues.Center };

tcp.Append(tcb);
//append the TableCellVerticalAlignment instance
tcp.Append(tcVA);
gCell.Append(tcp);

One thing to note is that the "After" line spacing will be set to 8pt so the above will actually look very similar to what you have with the default height:

But if you were to expand the cell you will see it's centred (almost, the spacing is still there throwing it out slightly):

To adjust the spacing, you can use the SpacingBetweenLines class:

TableCellProperties tcp = new TableCellProperties();

TableCellBorders tcb = new TableCellBorders(
        new TopBorder() { Val = new EnumValue<BorderValues>(BorderValues.BasicThinLines) },
        new BottomBorder() { Val = new EnumValue<BorderValues>(BorderValues.Double) });

//set the alignment to "Center"
TableCellVerticalAlignment tcVA = new TableCellVerticalAlignment() { Val = TableVerticalAlignmentValues.Center };

tcp.Append(tcb);
tcp.Append(tcVA);
gCell.Append(tcp);

Paragraph para = gCell.AppendChild(new Paragraph());

//Adjust the spacing between lines
ParagraphProperties paraProps = new ParagraphProperties();
SpacingBetweenLines spacing = new SpacingBetweenLines() { After = "0" };
paraProps.SpacingBetweenLines = spacing;
para.ParagraphProperties = paraProps;

Run run = para.AppendChild(new Run());
run.AppendChild(new Text("28.329,46 €"));

This gives this result by default:

And expanding the cell shows it really is now in the center: