===== get active sheet =====

===== get sheet by GID =====
<code javascript>
function mainDeal() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sourceSheet = getSheetByGid(ss, <PUT GID HERE>);
}

function getSheetByGid(spreadsheet, gid){
    gid = +gid || 0;
    var res_ = undefined;
    var sheets_ = spreadsheet.getSheets();
    for(var i = sheets_.length; i--; ){
        if(sheets_[i].getSheetId() === gid){
            res_ = sheets_[i];
            break;
        }
    }
    return res_;
}
</code>