Gimp is very good and important for me. But the graphics of a mandala you can do much better with a vector graphics program like inkscape.
You will find countless tutorials on the net and be thrilled.
Have the courage to work with inkscape, Gimp will not suffer because of it.
As @patdavid says, JPEG files can’t record transparency.
There is another problem. If you look carefully between the white and black areas, pixels are various shades of gray. If you simply make white pixels transparent, the gray pixels will remain, creating a sort of light halo around the dark areas. This may, or may not, be a problem.
If it is a problem, there are many possible solutions. One is to regard the black-gray-white image as transparency, which we can negate to be opacity, and then make all the pixels black (but with varying transparency).
I expect that can be done with Gimp. I know ImageMagick better than Gimp, so here is an IM command (Windows syntax).
magick lisel102.png -negate ( +clone -fill Black -colorize 100 ) +swap -alpha off -compose CopyOpacity -composite out.png
If your browser background is black, the black lines on a black background won’t show well. But any other colour will look good, with no “light halos”.
I know it is not the method you asked for, but have you considered converting to an SVG via a free online converter? SVGs are vector based so you can scale it as large as want an not get any pixilation or jpg artifacts.
Yes, your question was how to make your mandalas with Gimp. That seems to be answered.
In addition, if you had an interest in Inkscape, here are a few simple mandalas I made for myself a few years ago. You can look at them with Inkscape and use them as you like, maybe you’ll get a taste for it after all …