You have at least seven cards in your graveyard and/or exile that's an instant or sorcery, or that has an Adeventure.
Steps
Activate
Gwenna by tapping it, adding ({U} magic symbol) ({U} magic symbol) that can only be spent to cast creature spells or to activate abilities of creatures.
Gwenna trigger, putting a +1/+1 counter on it and untapping it.
Activate
Gwenna by tapping it, adding ({G} magic symbol) ({G} magic symbol) that can only be spent to cast creature spells or to activate abilities of creatures.
Cast
Ghalta by paying ({G} magic symbol) ({G} magic symbol) .
Brinelin and
Gwenna trigger.
Resolve the
Brinelin trigger, returning
Sailors' Bane from the battlefield to your hand.
Resolve the
Gwenna trigger, putting a +1/+1 counter on it and untapping it.