how do i create a downloadable gallery with image descriptions?

This is not a bug, it’s a style matter (that maybe renders the Question off-topic).

Using the Nextgen Download Gallery plugin, this can be solved by simply applying the following rule to your theme’s style.css file:

.ngg-gallery-thumbnail {
    width: 120px;
}

Adjusting the width to match your theme’s design.

And even the following rule, if there are incredibly large words, like Pneumonoultramicroscopicsilicovolcanoconiosis…

.ngg-gallery-thumbnail label span {
    word-wrap: break-word;
}