How to change the height BETWEEN sections in GROUPED UITableView?

On iOS 15 you can change the section header top padding

if #available(iOS 15.0, *) {
    tableView.sectionHeaderTopPadding = 0
}

You could use footer views between each of your sections and give them the size of the space you desire by implementing the following methods in your UITableView's delegate.

- (UIView *)tableView:(UITableView *)tableView viewForFooterInSection:(NSInteger)section

- (CGFloat)tableView:(UITableView *)tableView heightForFooterInSection:(NSInteger)section

In the first delegate method you can return a simple UIView instance, and in the second you can return the height that this view should be. This second delegate method should allow you to control the gap between sections.