Alle vs. ganz
While both German words alle and ganz can relate to the concept of all in English, they are not interchangeable. The core difference lies in whether you are talking about every individual member of a group (alle) or the complete entirety of a single thing (ganz).
Alle
A1alle means all or everyone and is used with plural nouns to refer to the total number of individual, countable items or people within a group.
Alle Kinder sind im Park.
(All the children are in the park.)
Ich habe alle meine Aufgaben erledigt.
(I have finished all my tasks.)
Sind alle da?
(Is everyone there?)
Wir treffen uns alle zwei Wochen.
(We meet every two weeks.)
Alle Wege führen nach Rom.
(All roads lead to Rome.)
Ganz
A1ganz means whole or entire and is used with singular nouns to describe the completeness of a single item or concept. As an adverb, it can also mean quite or very.
Er hat den ganzen Kuchen alleine gegessen.
(He ate the entire cake by himself.)
Ich habe die ganze Nacht nicht geschlafen.
(I did not sleep the whole night.)
Ganz Deutschland spricht über das Spiel.
(The whole of Germany is talking about the game.)
Das Auto ist noch ganz neu.
(The car is still very new.)
Die Prüfung war ganz schön schwer.
(The exam was quite difficult.)
Mir ist das nicht ganz klar.
(That is not entirely clear to me.)
Summary
In essence, use alle when you can count the individual parts of a group (all the apples, all the people). Use ganz when referring to a single, complete entity (the whole apple, the whole day). Remember that only ganz can also function as an adverb meaning quite or very, a role alle never has.