Hi Riccardo,
Currently we dont have any option to provide spacing between GroupBarItem''s icon and text.
As a workaround, please try giving space in the text property of GroupBarItems.
this.groupBar.groupBarItems[0].Text = " Item1";
Please take a look at the attached sample,and let me know if this helps you.
GroupBar.zip
We appreciate your interest in Syncfusion products.
Best Regards,
Seetha