Chris,
Any good quality cleaner will work prior to applying P21S Carnauba. Whether or not you use SMR or GEPC before waxing will depend on whether or not you're happy with the amount swirls/surface defects your car has. The GEPC is more like a glaze with mild cleaners that doesn't do much in terms of removing swirls. What it does do however is give your car a brilliant shine and a nice slick surface to apply the carnauba wax.
Also, in my experience, if you use the GEPC, Meguiar's #7 really isn't necessary. It doesn't increase the glossiness that much (nor fill in any more swirls). Plus, the GEPC is MUCH easier to use than #7.
So to answer your original question, while the GEPC isn't absolutely necessary, I would highly recommend it because not only is it very easy to use, it also produces excellent results. The only exception would be if you plan to remove swirls using Meguiar's #9 SMR 2.0. (or a similar swirl-remover that has a high oil content). If that's the case, then you can just go straight to the carnauba after the SMR.
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