no additional plugin is needed, to save as jpg, if gimp was correctly installed is only needed to add .jpg after the image name in the Save AS or " Save Copy as " dialogs
(at least if you did not change default save option)
As example if your want call your image "my-image" to save as jpg you just wrote there " "my-image.jpg "
If then you get message about missing plugin then is probable you downloaded a corrupt installer
How do I save a .xcf file as a jpg/JPEG?
Ok... But then where can I
Just download GIMP once