The Graphic® Save option sends the current version of your graph (with any changes: additions, re-positioning or deletions of graph elements) back to the ABAP program that called SAP Gantt Chart. The ABAP program then processes the new version further, as required for your application. This processing may or may not include actually saving the new version in the SAP System.
If you close SAP Gantt Chart without selecting Graphic ® Save, your changes remain local to SAP Gantt Chart, and are thrown away when you leave. Any further processing by the ABAP program will use the old version of the graphic (before you made your changes).