--I'm not a coder-- but I mixed code from a couple of answers and got this to work in the updated 2019 SS. Note: It does not work in the preview and you have to view your site from an incognito window (aka signed out of the backend to see it). PLUS you need to add this to the page header code injection section. If the page is in an Index, you need to add it to the Index's header code injection.
<script>
Y.on('domready', function () {
var galleries = Y.Squarespace.GalleryManager.getGalleries();
for( i = 0; i < galleries.length; i++){
galleries[i].setAttrs({
"autoplay": true,
"autoplayOptions": {
"timeout":10000,
"randomize": true
}
})
}
});
</script>