@melza0 – Without the specifics of the display type you are referring to I cannot say for certain especially without a URL to where it is inserted at although the following custom CSS added under Gallery > Other Options > Styles generally works for the Basic Thumbnails display (you may need to tweak it depending on any other CSS from the site):
/** Center Basic Thumbnails */
.ngg-galleryoverview {
text-align: center !important;
}
.ngg-gallery-thumbnail-box {
float: none !important;
display: inline-block !important;
}
Thanks!
– Cais.