How would I translate "the Departing" in German?
As in "You all, the Departing, give up all hope!"
I doubt "Departing" in this context is a gerund, so how would I translate this verb-turned-into-a-noun?
As no-one came up with a better idea I'll cast my comments into an answer.
... how to translate a verb turned into a noun?: take the present participle of the verb and add -e/-er/-en depending on case, gender and article:
Verb scheiden >> participle scheidend >> noun der/die Scheidende (sg.), die Scheidenden (pl.)
"You all, the Departing, give up all hope!" = Ihr, die Scheidenden - gebt alle Hoffnung auf!
Or use a verb as in Ihr, die ihr fortgeht/von uns geht/uns verlasst..., see comments above.
You'll frequently find participles turned into nouns as titles of works of art:
Die Stehende von xyz. = Standing woman by xyz.
Note that in English, you'll have to add a noun to make clear who is standing here. In German, this is expressed by the article: der Stehende / die Stehende / die Stehenden.