In this case I would use the star paint brush which comes with GIMP (it's called star, click on that "button" near "2. Star" in the screenshot it's a star, you might have a rounded brush),
and set the foreground color up to that specific flag's yellow
orientation "angle" ~53 to have the top branch of the star straight up, and play with the different size of the brush for the different stars of the flag
and set the foreground color up to that specific flag's yellow
orientation "angle" ~53 to have the top branch of the star straight up, and play with the different size of the brush for the different stars of the flag