Using gallery2 customfields via embed.php

I am using G2 embed.php to use Gallery API. I need to change customfields on my entity. We have used embed correctly for working on everything else, we just need to set customfields? Can someone kindly post how to change customfields using GalleryAPI?

I have managed to do several other things using embed.php like creating albums, importing photos, etc. I need some help with this!

Cheers!