Spline Link support in DHTMLX Gantt

Is there any possibility in DHTMLX Gantt library to show case the Link as a spline instead of straight square shape lines ?

Unfortunately, there is no built-in way to change the shape of the links, but it could be implemented in a custom solution. One of the options is to use SVG images of the custom links with addTaskLayer method. There is an article about it:
You need to know start and end coordinates of the source and target tasks. You can get it using posFromDate and getTaskPosition methods:
For example, it can look like this:

let startx = gantt.posFromDate(task.end_date);
let starty = gantt.getTaskPosition(task, task.start_date, task.end_date).top + 16;
let endx = gantt.posFromDate(target_task.start_date)
let endy = (gantt.getTaskPosition(target_task, target_task.start_date, 
target_task.end_date).top + gantt.config.row_height / 2);

Also, you need to create a function to repaint your custom links after changing in chart. You can use batchUpdate method to update multiple tasks/links at once:
For example, it can look like this:

gantt.batchUpdate(function () {
        for (let i = 0; i < target_links.length; i++) {
            let link = gantt.getLink(target_links[i]);
            let source_id = link.source;

Please check the example of how it could be implemented with Finish-to-Start links: