I've moved this from the poorly named Community Help forum (which is the forum for issues using the forums) to the InDesign forum so that proper help can be offered.
When the Export JPEG dialog box comes up with a bunch of options, what are the settings for:
Did you use a spot colour in your gradient? That could be the issue.
Thanks for reaching out. I hope your issue is resolved now. If it is, we'd appreciate if you can share the solution with us so that it can help other users with similar concern.
If not, please share the information asked in earlier responses. Looking forward to your response.