I'm working on addressing this for the next release. There are some strange conditions that will prevent it Friendica from showing a gallery that I have not figured out how to address:
1. Do not put ANY text *after* your photos.
2. Do not put any text *between* your photos.
3. Do not put any line-breaks/carriage returns between your photos.
It pretty much has to be an uninterrupted set of photos with no spaces or anything between them. Or it will break the gallery layout. It happens so often for people that I'm also doing a workaround to sort-of gallery the images anyway, even Friendica won't put it in a gallery.
I'm also working on making the post preview show you your images as a gallery too. Right now it only shows them full-width one after the other. So you have no way of knowing if you've done something to "break" the gallery layout or not until you've already posted it.
Here's the GitHub thread about it:
github.com/friendica/friendica…
I'm still trying to get my PR together for this. I'm also adding functional ALT tags to images and previews in the post composer.