I think you said somewhere that you are coming from Ad*be Illustrator. Gimp is not a vector editor although you can import svg files from other programs. Truthfully you are better off using Inkscape for that type of shape.
Using Gimp, the paint tool has a 'smooth stroke' option.
Then to thin the ends of a line In the Filters -> Distorts menu the IWarp tool.
This in a Gimp 2.8 (2.10 is easier, the Warp tool works on the canvas, but otherwise much the same)
https://i.imgur.com/9H7O0ZQ.mp4 40 second rough demo
Using Gimp, the paint tool has a 'smooth stroke' option.
Then to thin the ends of a line In the Filters -> Distorts menu the IWarp tool.
This in a Gimp 2.8 (2.10 is easier, the Warp tool works on the canvas, but otherwise much the same)
https://i.imgur.com/9H7O0ZQ.mp4 40 second rough demo