I got your point, you're settings NSP GK5 module to retrieve the first image inside article content to show up on frontend. Inside the article content, you add text next to the image as caption but Joomla doesn't know this is caption of image, it will consider it as other normal text in article so it will display there.
Joomla provides option called Images and Links so that you can add Intro images / Full article images + captions for each article. You can enable this option under Articles Options
Only when you add caption for intro text image / full article image for each article, Joomla will know it's caption of image.
It's normal behavior of Joomla 🙂
