Which Galaxy Tab? Anyway, on my S7 Edge with Nougat, in the gallery, open an album, then tap the 3 dots on the upper right to get to slideshow settings.
It's Galaxy Tab A , Android 7.. Galaxy S7 slideshow is working fine and I can access its settings. The tablet however was updated a couple days ago and they took away the settings and replaced it with "story" {Instagram style} it doesn't control time of slideshow or face zooming.