The "tab size" option in gedit only changes how tabs are displayed in the editor window. It has no effect on how the tab character
\t gets displayed in the terminal window when you run the program.
It seems to be possible to change the display width of a tab character in your shell environment but I wouldn't recommend it and it won't help you to successfully complete this problem set. The grader doesn't compare your output visually because it's an automated program – even if you got the tabs to appear to be identical to spaces in your copy of the appliance, the grader is only going to compare the raw output string to see if it's identical to the output that it expects. It's going to see
\t\t\t\t\t\t\t##\n when it expects a string with space characters instead of tabs, so it's going to mark the output as incorrect.