In odoo js framework you could change any widget class by using mostly 2 ways.

1- Using include function that it will change the properties(functions included) original class widget prototype

2- Using extend function that it will return a copy of the changes to the properties(functions included) original class widget prototype

In both cases you always will be able to access the overrides functions in the new ones using this.super or this._super (in newer versions) depending on the Odoo version. You could call it passing the same parameters received or just call it as using js prototype function apply to call it with arguments like:

 this._super.apply(this, arguments);

That is for example the case when you need to change a function like in your case but still need to call the original one and you could do it using the super call.

For the non widget classes created using js prototype like the case of GanttTask you could do what you are doing copying the original function to call it when you need to and just replace the original function in the class protoype but you need to put all your code inside the js odoo module like:

openerp.gantt_view_custom = function(instance){
    instance.gantt_view_custom = {};
    var module = instance.web_gantt;
    var _super_ = GanttTask.prototype.create;
    
    GanttTask.prototype.create = function(){
        var containerTasks = this.Chart.oData.firstChild;
        var containerNames = null;
        if (this.Chart._showTreePanel) containerNames = this.Chart.panelNames.firstChild;
        var predecessorTask = this.TaskInfo.PredecessorTask;
        var parentTask = this.TaskInfo.ParentTask;
        var isCParentTask = (this.TaskInfo.ChildTasks.length > 0);
        var self = this;
        
        this.cTaskItem = [];
        this.cTaskNameItem = [];
        
        //creation arrTasks
        if (!parentTask) {
            if (this.TaskInfo.previousParentTask) {
                this.previousParentTask = this.Project.getTaskById(this.TaskInfo.previousParentTask.Id);
                var lastChildTask = this.Chart.getLastChildTask(this.previousParentTask);
                this.posY = this.Project.posY;
                this.previousParentTask.nextParentTask = this;
            
            } else {
                this.posY = this.Project.posY;
            }
        }
        
        if (parentTask) {
            var task = this.Project.getTaskById(this.TaskInfo.ParentTask.Id);
            this.parentTask = task;
            
            if (this.TaskInfo.previousChildTask) {
                this.previousChildTask = this.Project.getTaskById(this.TaskInfo.previousChildTask.Id);
                var lastChildTask = this.Chart.getLastChildTask(this.previousChildTask);
                this.posY = parseInt(lastChildTask.cTaskItem[0].style.top) + this.Chart.heightTaskItem + 11;
                this.previousChildTask.nextChildTask = this;
            
            } else {
                this.posY = parseInt(task.cTaskItem[0].style.top) + this.Chart.heightTaskItem + 11;
            }
            task.childTask.push(this);
        }
        
        if (predecessorTask) {
            var task = this.Project.getTaskById(predecessorTask.Id);
            this.predTask = task;
            task.childPredTask.push(this);
        }
        
        //creation task item
        this.cTaskItem.push(this.createTaskItem());
        containerTasks.appendChild(this.cTaskItem[0]);
        
        if (this.Chart.panelNames) {
            this.cTaskNameItem.push(this.createTaskNameItem(isCParentTask));
            this.Chart.panelNames.firstChild.appendChild(this.cTaskNameItem[0]);
        }
        
        if (this.Chart.isShowDescTask) {
            containerTasks.appendChild(this.createTaskDescItem());
        }
        
        //Create Connecting Lines
        var arrConnectingLines = [];
        if (predecessorTask) arrConnectingLines = this.createConnectingLinesDS();
        this.cTaskItem.push(arrConnectingLines);
        
        if (this.Chart.panelNames) {
            //Create Connecting Lines
            var arrConnectingLinesNames = [];
            if (parentTask) {
                this.cTaskNameItem[0].style.left = parseInt(this.parentTask.cTaskNameItem[0].style.left) + 15 + "px";
                arrConnectingLinesNames = this.createConnectingLinesPN();
            }
            this.checkWidthTaskNameItem();
            
            var treeImg = null;
            if (isCParentTask) treeImg = this.createTreeImg();
            
            this.cTaskNameItem.push(arrConnectingLinesNames);
            this.cTaskNameItem.push(treeImg);
        }
        this.addDayInPanelTime();
        //    console.log("thissssssssssdddddddddddddssssssss",this)
        console.log("thissssssssssdddddddddddddssssssss",this.Project.posY)
        //    console.log("thissssssssssdddddddddddddssssssss",this.posY)
        //    this.posY = this.Project.posY
        //    console.log("thissssssssssdddddddddddddssssssss",this.posY)
        //    
        // ########################   My code
        if (this.Chart.Project){
            var project_obj = this.Chart.Project;
        }
        for (var key in project_obj) {
            if (project_obj.hasOwnProperty(key)) {
                console.log(key + " -> " , project_obj[key]['Id']);
                var id_project = project_obj[key]['Id']
                $("#"+id_project.toString()).css("display", "None");
            }
        }
        $(".taskNameItem").css("display", "None");
        //hiding the workorder
        // my code
        return this;
    }
});

Hope this helps