Knowing the Invocation metamagic lets you summon great form spirits. In addition to the standard and optional powers for their spirit type, great spirits start with an additional great spirit power according to the "Great Spirit Powers" table in Street Wyrd (p.66). Depending on what type of spirit you summon, this great form power could be Storm (Air, Fire, Guardian, Water), Paralyzing Howl (Beast), Quake (Earth), Temporal Displacement (Guidance), Magical Guard (Kin), Regeneration (Plant), or Wealth (Task).
There are three new spirit powers listed in Street Wyrd that are available to great form spirits but are not assigned to a spirit type in the "Great Spirit Powers" table: Aura Masking, Divining, and Endowment. I would treat these as additional optional powers for great form spirits. Remember, all spirits get one optional power for every three full points of Force (SR6 p.147).
Note that Astral Gateway (SW p.66) is only available to free spirits, not great form spirits.