@Anonymous Ist you will need some variables to determine the column to be sorted on and the direction of the Sort. For example, for the Start column, in the OnSelect of that up/down icon, you have to have,
Set(varSortColumn, "Start")
Set(varStartSortAscending, !varStartSortAscending).
You have to do the same for the other 2 columns (Agent & Date)
In the Items property of the Gallery,
Switch(varSortColumn,
"Start", Sort(ColGridData, Start.Value, If(varStartSortAscending, Ascending, Descending),
"Agent", Sort(ColGridData, Agent.Value, If(varAgentSortAscending, Ascending, Descending),
"Date", Sort(ColGridData, Date.Value, If(varStartSortAscending, Ascending, Descending),
)
