Music production and mixing are two different things. Mixers perform a completely separate role.
Garageband does have an in-built tuner but it's not nearly as good as other tuners, including plug-ins, pedals, applications, and more.
Producers and composers are fundamentally different from each other in a number of ways.
Recording acoustic guitar in Garageband (or any DAW) is very easy. Don't worry.
What's the difference between a music producer and a DJ, and where does the confusion come from?
How are music producers and audio engineers different from each other? This article touches on the roles these two professionals play.