Interface

Yes it is possible.

You can add an extra gridRowLayout() outside for a!forEach and use the sum() function on the respective column. 

Something like this you can build.

a!localVariables(
  local!gridData: a!map(
    col1: "",
    col2: "",
    col3: "",
    col4: "",
    col5: "",
    col6: ""
  ),
  a!gridLayout(
    headerCells: {
      a!gridLayoutHeaderCell(label: "Col1"),
      a!gridLayoutHeaderCell(label: "Col2"),
      a!gridLayoutHeaderCell(label: "Col3"),
      a!gridLayoutHeaderCell(label: "Col4"),
      a!gridLayoutHeaderCell(label: "Col5"),
      a!gridLayoutHeaderCell(label: "Col6")
    },
    addRowLink: a!dynamicLink(
      label: "Add",
      saveInto: a!save(
        local!gridData,
        append(
          local!gridData,
          a!map(
            col1: "",
            col2: "",
            col3: "",
            col4: "",
            col5: "",
            col6: ""
          )
        )
      )
    ),
    rows: a!forEach(
      items: local!gridData,
      expression: if(
        fv!index = count(local!gridData),
        a!gridRowLayout(
          contents: {
            a!textField(readOnly: true()),
            a!textField(readOnly: true()),
            a!textField(readOnly: true(), value: "Total"),
            a!textField(
              readOnly: true(),
              value: sum(index(local!gridData, "col4", {}))
            ),
            a!textField(
              readOnly: true(),
              value: sum(index(local!gridData, "col5", {}))
            ),
            a!textField(
              readOnly: true(),
              value: sum(index(local!gridData, "col6", {}))
            )
          }
        ),
        a!gridRowLayout(
          contents: {
            a!textField(
              value: fv!item.col1,
              saveInto: fv!item.col1
            ),
            a!textField(
              value: fv!item.col2,
              saveInto: fv!item.col2
            ),
            a!textField(
              value: fv!item.col3,
              saveInto: fv!item.col3
            ),
            a!textField(
              value: fv!item.col4,
              saveInto: fv!item.col4
            ),
            a!textField(
              value: fv!item.col5,
              saveInto: fv!item.col5
            ),
            a!textField(
              value: fv!item.col6,
              saveInto: fv!item.col6
            )
          }
        )
      )
    )
  )
)