For just a conversion, using Gimp is overkill. The CLI tool of choice for image conversions is ImageMagick. Can be as simple as:
There are of course plenty of options to fine-tune the conversion if necessary.
You can even do a while directory of images like this:
(some more ways here).
Code:
magick convert image.heic image.jpg
You can even do a while directory of images like this:
Code:
convert *.heic -set filename:base "%[basename]" "%[filename:base].jpg"
(some more ways here).