I would like to embed the free version of ACF, but, if my understanding is correct ACF is licensed as GPL. GPL requires that if I include ACF in my premium plugin, then my premium plugin must also be GPL.
Yes that would be a correct understanding of the GPL.
Aside from that though, even if you don’t include ACF, as John stated it is quite possible your plugin could be judged to be a derivative of WordPress… which would mean it has to be GPL’ed anyway.
However, as I’ve written here, I think the statements in the article on including ACF in a plugin also contravene the GPL.
