Gantry5 - custom field type for particle configuration

I am new to the Gantry5 Framework and I started out creating a custom Particle, wasn't that difficult all in all.

I am using Gantry5 for Wordpress and I would like to create a particle, which can be configured with an existing Page.

Therefore I need a form field, in best case a dropdown Menu, which displays a list of pages.

Right now the configuration of the Particle (custom/particle/page_particle.yaml) looks like this:

name: Page
description: Displays the content of a Page
type: particle

form:
  fields:
    enabled:
      type: input.checkbox
      label: Enabled
      description: Globally enable to the particles.
      default: true

    title:
      type: input.text
      label: Title
      description: Customize the section title text.

#following lines describe what I need
    page:
      type: ##custom type
      label: Page
      default: -1

Does anybody know how to add the nessesary code (where to place, how to name) and how to tell Gantry about that new Type?

Thanks in Ahead!

via Chebli Mohamed