You have not completed nor are currently venturing in Tomb of Annihilation.
Gwenna does not have summoning sickness.
You have a creature card exiled by Semblance Anvil.
Steps
Activate Gwenna by tapping it, adding ({1} magic symbol) ({B} magic symbol) that can only be spent to cast creature spells or activate abilities of creatures.
Cast Acererak by paying ({B} magic symbol) .
Gwenna triggers, putting a +1/+1 counter on it and untapping it.
Acererak enters the battlefield, returning itself from the battlefield to your hand and causing you to venture into Lost Mine of Phandelver.