Anything that is related to the term "Zone" (as opposed to "Zone" in composite words, or combined with numbers or other qualifiers of any kind) in German is somehow "loaded" by German history - At least in Western Germany, "Die Zone" used to be the term for the Soviet-occupied zone and later, the German Democratic Republic.
So
Dann geh doch in die Zone!
would have been used towards people being considered "overly socialistic" or communist to tell them they might probably better fit there.
Thus, you wouldn't want to use anything close to a literal translation.
An expression that would convey the meaning of someone being "in the Zone", i.e. exactly within his range of abilities and skill would be
Er ist in seinem Element
and, more or less related to that and a bit more sloppy
Er fühlt sich wie ein Fisch im Wasser
In case you are relating to "being in the zone" as to the common meaning of "being in the flow", you might want to maybe revert to a literal translation of that. German has some idioms that somehow relate to that, but not describing a person's state, but rather the flow of actions.
Jetzt geraten die Dinge in Fluss
Would maybe translate to "now we get things rolling"
And "the Flow" as a psychological technical term is used in German as well, as a loan word, sometimes translated as "Schaffens-/Tätigkeitsrausch"
