Hi Wolf,
Particles is not a Joomla extension that why you can not install it via Joomla extension manager. You need to copy files manually into your theme directory to use the particles via Gantry 5 layout.
See concept http://docs.gantry.org/gantry5/particles
Gantry 5 doc http://docs.gantry.org/gantry5/tutorials/copying-particles
Our document for particle implementation https://joomlead.com/gantry-5-particles/particles-documentation/#copy-particles

“The Light Gallery in the Double Template has only a limited choice of options; where is the Light Gallery with all options?
”
The light gallery from double theme is 1.x version, you can implement the 2x version for more options.