In Power BI Report Builder, grid lines in a Matrix are controlled at the cell and row level, not by a single “gridlines” toggle like in Power BI Desktop.
To show both vertical and horizontal grid lines, select the entire matrix first (click the grey handle in the top-left corner), then in the Properties pane set BorderStyle = Solid, BorderWidth = 1pt, and choose a BorderColor. This applies an outer border only.
Next, you need to apply borders to the cells themselves. Click inside the matrix, select all data cells, row headers, and column headers (you can drag to multi-select), then set Left, Right, Top, and Bottom borders to Solid with the same width and color. This is what actually creates the internal grid lines.
If some lines are still missing, check the Row Groups and Column Groups. Select each group (including totals) and ensure the border settings are applied there as well, since grouped rows can override cell borders.
Once this is done, the matrix will render with clear vertical and horizontal grid lines in the report output.
