Try to change the gradient color > click on that gradient bar in gfig, a new window opens, change the gradient ➤ don't click OK yet ➤ click again on your favorite gradient, now Click OK, you go back to the main gfig and try again to draw you triangle.
This might happen after some Ctrl+Z (undo) inside gfig
On a side note, for everything equilateral, stars, polygons, I would recommend to draw them with the Symmetry Painting tool in "Mandala" mode, (if not showing in your docks go to Windows ➤ Dockable Dialogs ➤ Symmetry Painting), you will get the full canvas view and better controls on the thickness of your brushes.. (just don't forget to put it in None mode after drawing
) ➤ https://docs.gimp.org/2.10/en/gimp-symmetry-dialog.html
This might happen after some Ctrl+Z (undo) inside gfig
On a side note, for everything equilateral, stars, polygons, I would recommend to draw them with the Symmetry Painting tool in "Mandala" mode, (if not showing in your docks go to Windows ➤ Dockable Dialogs ➤ Symmetry Painting), you will get the full canvas view and better controls on the thickness of your brushes.. (just don't forget to put it in None mode after drawing
) ➤ https://docs.gimp.org/2.10/en/gimp-symmetry-dialog.html