How i can provide bottom space between two groups

something like this i want and i want to fill the space with color

Hello Nitish,
Right now, there is no built-in way to do that. You need to implement a custom solution by using the Gantt API, Javascript, and CSS.
For example, you can add an unscheduled task and add the style rules to show the horizontal line on the row.
Here is an example of how it might be implemented:

The dev team will add that feature in the future, but I cannot give you any ETA.