Thanks for the question. As you are new to the harmonica, I’m assuming you have not started bending yet. This means that you can only play in the key that the harmonica was designed for. So yes, when playing with songs in G you will need a G harmonica.
However, as it turns out after you do learn bending the main key you will be playing in on your C harmonica is G!! This is explained in step 9 after the bending is under control.
In any case it won’t hurt to grab a G harmonica and use both to play with 🙂
Hope that helps!